40x40 if you want the multi-rank look, which is fine for WAB/CoE/Hail Caesar, or 60x20 if you want the WRG/DBx/FoG 'standard' (you can fit 3 figures on a 60x20 base, and none of those rules actually care how many figures are on the base).
I went for the latter as it lets you play more rules with the figures (including WAB/Hail Caesar), but it depends what rules you are planning on using them for.