As has been said already, depends on the rules. Majority of rule sets define a base width, to do with ground scale and the frontages of units to the chosen scale. More important in eras of linear tactics, like say, Napoleonic when a batallion of infantry drawn up in line occupied a given area of ground. For example, all the DBA rules (DBA, DBMM, DBR, HOTT) specify 40mm width for 15mm or less figures and 60mm width 25/28/20mm, depth is less important but it features in the rules as units recoil from combat by their base depth.
'Irregular Wars' rules only uses one base size for each figure size range, i.e. 40mm square for the 15mm and less and 60mm square for 20mm and above.
It very much depends I think, on how you want your armies to look. Either dense pack on large bases or smaller formations on smaller bases. Base size is really out of your control if you play against others using a given set of rules.
If you are a solo gamer then do what you like.