Why base at all.Though most historical figures come with some sort of base as part of the figure these bases are often inadequate to the task of supporting the figure because the truth is these are not really primarily bases but rather reservoirs for the metal used in the casting to ensure that the rest of the figure fills properly you therefore need to stick the figure to something so that it will stand up & given that you have to base it looks better where possible to use similar sized & shaped bases for a group of figures. However there are other considerations the first to me is the scale of the figure itself & the second is the densest fighting formation that you expect the troops to be in also the closeness of the terrain you are likely to use. The final thing is what is avalible to use. I think its a bit easier to give examples.
28mm figures for Dux Britannarium dark ages skirmishing
20mm figures for TW&T & IABSM3 WWII platoon to company level actions
15mm figures for M&B WWI platoon to company level actions
28mm figures for A Hundred years earlier Smalll actions form the Nine years war of the 1690's
to be continued....
Ginger