Techs & Trends
HTC Touch Pro and Micro SDHC
From the website(www.htc.com):
Expansion Slot microSD┞¢ memory card (SD 2.0 compatible)
SD 2.0 is the software version that allows SDHC cards to be read and rewritten. It is also reverse compatible, so any regular microSD cards will work with it as well.
The phone is already written to support up to 32gb according to HTC, but nobody can really test that yet since there are very few 32gb cards floating around... You will be just fine with a 16gb or smaller, and when 32gb become available and affordable, that should work too.
And FYI, WinMo phones can have a .cad file loaded and executed that converts them over to SD 2.0 compatible if they aren't already. Check out XDA-developers and do a search for it.