I have a steel Trek MTB that looks to have the same size tubing. If your LBS can't measure the post for you, I suppose you could make the educated guess of 26.8. If it fits, you're in. If it doesn't, you'll know what you need.

I thought that size tubing would be right for a 31.8mm clamp, but as that seems too small, it must be 34.9mm.