I purchased a road bike ( parlee z5 ) last year and had a fit done and have since settled on a 90mm stem. Also have a 25 mm setback post where seat is clamped at the back of rails ( slid forward? ) Bike feels fine but reading many posts on this forum I can't help but think because it seems most bike specs forum members post have 100-130 stems if my frame size is not ideal for me.

I am 5'6" and the bike has 52.5 eff TT. Wondering if the 51" TT size (XS) would make a difference for me or is it just aesthetics ?

A 90mm stem on a 51cm bike is completely appropriate. This site is generally biased towards smaller frames for a given height, slammed stems, extra long stems.
The only item you may want to consider changing is the seatpost. A zero-setback post would clamp the saddle rails in the middle of the range for your given position, which is generally stronger. It also looks better!
This all does not mean your bike is NOT too big, rather with the given information, I would not immediately jump to that conclusion.

You'd probably fit better length wise on the 51. However, depending on how flexible you are, you need to watch HT length and also wheelbase and angles.

If you are happy the way the bike handles and are happy with your saddle to bar drop, I would stick with what you have. A nine stem is ok. A ten or eleven would be nicer aesthetically but then you might end up with a stack of spacers so its not straightforward.

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Not everything that counts can be counted. Not everything that can be counted counts.

If it feels good, ride it.

People here get way to caught up in the aesthetics of bike fits. And by that I mean, they prefer a traditional amount of setback, with really low bars and a long stem. I can understand when a person posts a picture of a bike and there is a glaring problem; i.e. seat forward on a straight seatpost, bars above seat, or stem under 80mm. Those to me are red flags, but still not necessarily wrong, just there should be a good reason for them and the person may be best served consulting a different fitter.

However, its stupid when people criticize other people's fits without actually seeing the person on the bike; or make sweeping generalizations like a bike with a stem under ____mm won't handle well. Maybe the bikes wheelbase and/or trail lends to better handling with certain length stems, maybe the person has their weight distibuted in such a way that the stem is better at a certain length, maybe they run a longer than average bar reach...

Handling, fit, comfort and proper sizing boil down to far more than what the average WW thinks looks good.

So to answer your question, maybe a smaller bike would be better; but its hard to know based on what you told us.

I'm only a 1/2 inch taller, with long legs - 83cm cycling inseam with 73cm saddle height. I use a 53cm TT with a 74.5 degree STA, 110mm stem and a post with 25-32mm of setback. With 25mm I need most saddles slammed all the way back. This saddle position is not based on a knee over pedal (KOP) measurement, but proper weight balance of the rider over the saddle. I'm probably 2cm behind KOP.

If I had your Z5, I might need a 120mm stem because the TT is shorter and the STA is only 74 degrees. Those combine to make the reach about 1cm shorter.

That said, I do use Easton EC90-SLX3 short reach bars. If I used longer reach bars, my stem would be 10mm shorter.

Before buying a new frame, you should experiment with the fit. First try moving the saddle further back, about 5mm at a time. That will reduce weight on the hands and tell you if a little more reach is feasible. After that, try a 10mm longer stem. IMO, fitters can only take you so far into the complete fitting process and many fitters are very conservative. If I were to get a fitting today, I'd probably end up with the saddle 2cm further forward, and about 4cm less sadle to bar drop (7cm instead of 11cm).

If you insist on buying a smaller frame, be careful not to change the head tube length or stack height by too much. Also don't forget to consider the STA. A steeper STA lengthens the reach by about 8-9mm per degree in this frame size. With some frames you may get a shorter TT, but give up much of the reduced reach with a steeper STA.

In the case of the Z5, you'll only lose about 4mm of reach with the 74.5 degree STA on the next size smaller frame.

DaveS, how can you make these recommendations without any more feedback from the rider or at least a picture?

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Did you read the rest of my post? I did acknowledge that their can be problems with fits, but a 90mm stem in my book isn't necessarily one of them.

What recommendations? You mean to experiment with your fit? IMO, accepting any fitter's position as the only proper one is stupid, particularly if all they did was follow the ancient idea of setting the saddle fore/aft position with a KOP measurement.

It costs little or nothing to experiment with changes to your fit. You can always put things back where they were, if things don't work out.

I've read dozens of similar posting over the last 10 years and usually find that most riders have shorter legs and longer torsos than I do, so it make even less sense that they need short stems.

I should have mentioned brake/shift levers too. Shimano has the greatest reach and Campy the least. I use Campy. If I was using standard reach bars and Shimano levers, I might use a 90mm stem too, instead of a 110mm.

After re-reading your post, I agree. Experimenting with setback is not a bad idea, and I agree that getting the seat back tends to lead to better weight distribution and actually encouraging a slightly longer reach.

Regarding saddle set back i'm an odd bod. I use an inline post with saddle tilted up at the front to get me where i wanna be. I'm 179cm with an 88.5cm inseam and 78.3cm saddle height. I've been all over the place regarding frame sizes and have settled with a 54.5cm tt. 95mm stem with handlebars having 85mm reach.
My saddle to bar drop is about 15.5cm and crank length is 177.5mm.
I got so tired of reaching out over the years on recommended sized frames and long stems i could not even imagine using the bar drop i have now. The bar drop allows clearance for climbing out of the saddle with arms comfortably positioned.
When seated my arms have also found comfort while lightly supporting my top half that is now much more relaxed than ever.

Thanks for feedback. Here is some more info. My saddle to bar drop is 6cm

Stem is 6 degree in down position with 20mm spacer underneath

Although the fitter used kips I later changed fore aft position using center of gravity approach as described by Keith bin target paper. Still making minor adjustments from time to time. Let me know if any other info might be useful.

Hrm I really need to proofread what I type... Well I think you get what I'm saying

The reason most guys choose a smaller frame and throw a 140mm stem on is that is the only way they can get the bar low enough. That does not seem to be your problem. I agree that with your saddle position and stem length you could use a shorter toptube, but think of how many spacers you would run...

Judging by bike appearance only, there is nothing that stands out to me as saying this is an ill fitting bike. Frankly, most people that have huge saddle/bar drops are lying to themselves. But hey, if it makes them feel better in their head, no big deal. your setup looks appropriate, balanced.
Keith Bin Target Paper instead of Keith Bontrager? Damn autocorrect...