A bit late to the party, but I'd add that they do not slant on the sides, so they are 25mm across the top as well. It's only a couple mm
's wider on the top that a regular GW
-style base, but it does make it so that some figures feet won't overhang the base as much.
Also, as well as being completely flat on one side the circular indent on the other (meant to accomodate matnic figs) can be good for hiding older style figures who have 15mm circular cast-on bases. Sometimes you have to shave the edges of the base or the inside of the mantic base, but it's easy enough to do.
Here's a brief article showing the mantic bases used with vintage "Metal Magic" brand figs.