H&R Napoleonic Spaniards I got recently were already painted, so I thought I'd make use of them to test out a proposed basing option. Here they are, along with trusty 28mm Sergeant Measureby, for scale.
The base size is 40 x 20 x 3 mm, which lets me mount two ranks, each of two strips of infantry without having to separate them and glue them individually, which is, in this scale, a pain in the arse to be frank. I did consider 20 x 20 mm bases, but most of the time they'd just be more fiddly than they're worth on the wargaming table. 40 x 40 bases were another option, but I'd have to base the troops in at least three, and probably four ranks or else they'd look lost in the middle of a vast plinth. I'd consider 40 x 40 mm bases for units that never go below three or four ranks (pike blocks, for example), but I prefer two-rank basing, which I can stack for attack column formations as needed.
This size base will allow 6 cavalry abreast, and probably two guns to represent a battery, though I may yet go to individual guns on 20 x 20 mm bases..
A 3mm thick base looks pretty massive for figures this small, but it makes the individual bases handleable without having to smush the figures up. These tiny troops are very easy to bend or break at the ankles, so utility triumphs over æsthetics for my purposes. Another advantage of the thicker base is that I can glue printed paper strips with unit information to the back, if I so desire.