[Imp]

For WWII the general consences seems to be 80x80 or 100x100
Change the depth as in front mainly dependant on number of units.
Width I would say if have a decent number of vehicles 100 or 120 seems to work but also take into account the following.
Dense terrain like urban or jungle you might want to shrink.
Open like desserts you might want to increase
Vision also plays a part if its 70 odd in a dessert you can often see the first unit that moves if its only 5 obviosly you can't.
For MBT as things get more modern & quite a lot faster with wheeled APCs having 30 odd MP I tend to increase the width.
Once SAMs & helos are possible nearly always play at the max as they are very quick & engagement ranges go up for everything except infantry, a kill at 2+km is not unusual so 80 wide in open terrain is not enough esp if its not a meeting.
Also might increase depth slightly as speed makes units more capable of darting from cover to cover & good radios allow a greater flexibility to the units.