http://www.warflag.com/shadow/cast/figuresize/figuresize1.jpg
Excellent comparison of various 25mm figures, albeit for Colonial figures. General rule of thumb is on the tabletop if you field units from the same company or of similar heft/size you should be fine. In skirmish, i.e. 1:1, or RPG scale it doesn't really matter so much as long as you don't mix 15mm with 25mm for example.
http://theminiaturespage.com/ref/scales.html
for a more general overview of miniature figure scales.
HTH,
Casey
Excellent comparison of various 25mm figures, albeit for Colonial figures. General rule of thumb is on the tabletop if you field units from the same company or of similar heft/size you should be fine. In skirmish, i.e. 1:1, or RPG scale it doesn't really matter so much as long as you don't mix 15mm with 25mm for example.

http://theminiaturespage.com/ref/scales.html
for a more general overview of miniature figure scales.
HTH,
Casey