Cezary,
It should probably conform to your local gaming group's basing conventions, so you might try asking them. In general, though, I'd imagine that as long as your bases allow you to cleanly determine front/flank/rear and it's not too difficult to tell units apart, anything you do will probably be fine. Lots of US folks use pennies which are probably 20mm across.
Our local group uses small Flames of War bases (1" x 1.25"

for infantry and vehicles. Infantry support, heavy guns, HQs and COs go on round / hex / oct bases (your preference), with IS and guns ~ 25 mm diam, HQs ~30mm, COs ~40mm.