My original thought was to go with 10mm, but when I tried this it definitely seemed on the small side, strange given that DBx uses a 40mm frontage for 4 figures! With the next convenient amount of mm being 15, I cut up a few of those. Now the problem was they seemed too big!
Here's a picture for comparison:
And here is another reason why I went off 10mm squares... my standard bearer just wouldn't stand up!:
Then I saw this post on TMP asking a similar question. The first reply caught my interest. 'Nazrat' said he used a 1/2" square as the basis for his WAB basing. It had never occurred to me to use inches... 1/2" is 13.7mm, not something I would want to measure out a hundred times using metric measurements! But the convenience of a half inch can not be denied. So I went ahead and cut out a base to see what it looked like (middle):
Seeing the results I am pretty much converted to the idea. Not too small and not overpowering, it seems a good fit to the figure size. So the next question is, what do I use for cavalry and larger monsters?!
Off the top of my head I can see the convenience of 3/4" and 1" frontages for them, but I'll wait and see when I get some cavalry for my first army.