I have raced the last couple of seasons on a frame with a 56 cm top tube and a 12 cm stem and the handling has felt good but I feel like I want to go a little longer. I've been looking at a cross frame with a 55.5 cm top tube, if I put a 13 cm stem with it the tt/stem lenghth will be 0.5 cm longer. How will this effect the handling, will it still be ok, or will I be too far out over the front wheel?

I say you'd be fine. But what can I say, I have a 54.5 TT and 14cm stem on the roadbike...

B.T.W. the total reach also highly depends on the type of handlebars you're using, some have more reach than others. What are you using?

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my plastic bikes, 29ers & stable of old steel

I'm using Ritchey Logic WCS II with 82 mm reach.

Unless you really need the drop that this size frame allows you (headtube length) I would choose to go up a size. A cross bike that small with a 130 mm stem is not going to handle well IMO, even the 120 mm stem on a cross bike smaller than 58 is questionable in my mind.

I feel that for the best handling the stem to toptube ratio for a cross bike is best kept a bit less than I would normally use on a road bike.

My bikes for reference:
Road has 575 TT, 120 stem
CX has 580 TT, 100 stem (could run 110)

Another factor is the seat tube angle. If the smaller frame has a different STA that will also affect the length of stem you need to achieve the fit you're looking for. I know it did on mine.

According to my experience, you will handle the bike much better with a shorter stem. 130cm is really too long, better to opt for another frame size

Thanks for the confirmation. I was thinking it'd be too short, but I could get a really good deal on the smaller frame so I was considering the option. But it sounds like even a longer top tube and shorter stem then what I have been racing will be better.

Yes, I agree that a bigger frame would be a better choice as well. I run a 140mm stem on my road bike, but much prefer a setup that lets me use a 100mm (110mm max) on my CX bikes.