I base on washers with a 1/4 inch bolt hole. The center hole of the washer is filled with epoxy and a 3/16th inch diameter by 1/32nd inch tall rare earth magnet so they can be easily transported on steel sheet lined boxes.
One of my primary goals was to be able to put these washers into a sabot base so I could use them for a wide variety of rules including single based skirmish plus formation battles like DBA basing, which leads me to prefer slightly smaller bases, so I can do things like fit two side by side on a 40mm frontage.
I use a variation on 1-2-3 basing. For most troops, I do a mixture of single and double based troops. Heavy, close formation infantry types tend to get more double bases, while lighter types lean more towards single based and true skirmishers are almost all single based. Three to a base is usually reserved for horde types or smaller figures like hobbits, faeries, or small animals.
Most often, I use a 16mm (5/8ths inch) OD washer for single man-sized or smaller troops.
19.5mm (3/4th inch, same as a penny) OD washers for 2 figures for formed infantry, or single large infantry. This includes champions, flags, extreme action poses, or larger than man sized troops like Orcs or smaller Ogre types. If the commander types for an army aren't particularly impressive, I may mount them plus a musician or flag bearer on a base like this.
25mm (1 inch) OD washers will fit 3 close formation infantry, or 1 significantly larger than man-sized troops. Large Ogres, trolls, smaller ents, smaller giants, etc.
37.5mm (1.5 inch) OD washers are reserved for the largest of monstrous infantry types. Very large trolls/ogres, giants, small dragons, etc.
Cavalry is usually on a 20mm for singles, or for heavy close formation cavalry like cataphracts, two on a 25mm washer. I've tried doing "pill" bases by gluing two adjacent smaller washers to a polystyrene sheet and then cutting them out. It looks decent, but in order to fit in standardized size sabot trays, you have to drop to 1/2 inch OD, which leaves them prone to tipping over on their side when not in the sabot. It's also quite a bit of work, and still doesn't quite fit the proper numbers for an "official" DBA base, so I've made my peace with simply using one less figure than recommended (1 for Light horse, 2 for Cavalry and Knights, 3 for Cataphracts).
I have some 30mm washers for particularly large or action-posed horses (at a full gallop, with tails flying and lances readied).
For larger monsters, modern and sci-fi weapon teams, small power-armor mechs, and other small vehicles, I tend to simply use whatever flat washer will fit them reasonably. Most rule sets I've encountered don't tend to care about close infantry formations and frontage for such units.
ponders getting some of his 28mm fantasy figures off the shelves and onto the tabletop.
