Jen,
These are the bases with the ring

These are GW style bases. It has a chamfered edge. The very bottom(widest part) is 25mm. The actual flat surface is less.

Here are Reaper Bases. The sides of the raised part is flat. Giving you almost 25mm of flat space.
Some examples of what I use.
Here is a GW bases with Swordswoman, The Ax guy is on a Fender Washer, and the Reaper Bones Beetles are on Fender Washers as well. I use an acrylic medium to give texture to the base and cover up the integral Reaper base.
Pick a base you like. Pick one that is easy to get and cheap. Doesn't matter for Frostgrave, just be consistent with your warbands. Makes playing less fiddly and looks better for a warband to have all the same bases. It helps tie the "look" together even when they might be different manufacturers.
Hope this helps.