I wear a 42.5 giro prolight slx and wonder if i can fit a medium shoe cover.

IME, Castelli shoe covers run small. I own the Narcisista, which is quite stretchy, and still had to go one size larger then what is recommended on their sizing chart. EM3

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My 2011 Cervelo R5

I am usually on the edge of a small shoe cover but in the Castelli Pioggia 3 I had to go to a Med. I wear a 41 and 90% of the time the small will fit better than a Med, except this time. Hope that helps.
I would add that I find their fit chart and temp ranges very accurate.

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The word "Brifter" should be banned.
Casati Vola SLi- viewtopic.php?f=10&t=108931" onclick="window.open(this.href);return false;

on s-works shoes, which aren't bulky, my size is at the upper end of the castelli chart, i find i have to go up a size sometimes, but it depends on the model+version of shoe cover, some are stretchier than others

which covers are you considering?

Same size shoe and I use the Large in Castelli

Well i actually already paid for them online. 25$ best price. They are medium castelli piogga 3s

I just bought Castelli Pioggia 2 covers on sale. With size 42 Specialized shoes, the size large covers are a very tight fit.

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http://climbinglama.blogspot.com.au

I went with what the sizing chart suggested and got a perfect pro fit on my Mavic Zxelliums. The Pioggias are quite hard to get on and not all that stretchy, you just have to take your time getting them on.

I do see quite a few people who appear to prefer oversized overshoes so I could understand a few people might prefer a larger size , so I'd recommend buying the correct as well as the larger size from somewhere like wiggle which offers free returns.

Castelli sizing is as unpredictable as an English weather
That what mrfish suggests is very sensible option when comes to decision what Castelli size you should get. I've got quite a lot of their garments and majority of them were purchased by going for 2 sizes with the return unwanted one.
Evanscycles, CRC the other two shops with free returns. In the Evans case you have to do that in one of their shops (excluding PayPal payments, which must be sent back at your cost).

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Cervelo R5 VWD 5637g/12.43lb
Cannyon Ultimate slx 5592g/12.33lb
Pronghorn XC SW Race 8284g/18.26lb