As
goose-2 said most land units can control hex, except Artillery and AA, but to be complete, some AA or Artillery can switch to AT mode (for ex, Flak 36, Russian Su-122). In AT mode those units can actually control flag hex. Use the Switch button or 'W' key to switch mode. Suppose you have a Flak 36 which by default is in AA mode, switch to AT mode by pressing 'W', deselect this unit by right clicking on an empty hex, then hex capture become effective. It is important to deselect the unit to make capture effective, then you can select the unit back and even switch it back to AA if you want.
Units size may vary from scenario to scenario, roughly division sized. However due to the one unit per hex nature of the game, there are some abstractions, as AA or AT units are separate from infantry or tanks, whereas IRL most units get some organic AA At or artillery, so there is not an easy direct matching of historical OoB.
For example the Grand Campaign features battle s at a zoomed in scale compared to the vanilla game. And even in the GC the size is not really constant, for example the Villers Bocage map features a close in view of M. Wittman attack in the city streets.
For Mods check those links:
viewtopic.php?f=147&t=39054
http://www.twcenter.net/forums/showthre ... nd-modding