Besides the calipers, you should check between the stays for tire clearance. Often it is the chain stay's that limit the width. Here is an example. This is a Pinarello Tre Crime that I picked up for my daughter. The tires are what was on it and they are 25c. I was surprised to find such small clearance. Compare with the fork too.
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P1020919 on Flickr[/IMG]
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P1020918 on Flickr[/IMG]