Don't know how many are in a division in the game, doesn't really matter though, units are counted at the division level.
Make sure you make good use of
HQ units and don't overload your attack beyond the leadership limits.
Unfortunately turtling strategies don't work so good if you ever plan to go on the offensive, defenders can use as many units as they have in the tile to defend with out penalties, while attacking forces are limited by the best commander's unit limit. The AI doesn't tend to attack foolishly from my expirence, so its diffficult to grind them down defensivly.
I would suggest you use blitzkrieg tactics, use armor+mech arty as your spearpoint to punch through advance and encircle, with trailing mechanized/motorized to secure the ground behind the spear, and use basic infantry to clean up the pockets, prefferably with CAS to soften up the enemy first.
Oh and the british navy will eat your pathetic fleet for tea the moment war starts.
Last I would not recommend hitting through the maginot line, that area is heavily fortified and you will need a significant numerical advantage to go through, with air power.
http://www.paradoxian.org/hoi2wiki/i...rmy_management
http://www.paradoxian.org/hoi2wiki/i...ssorted_Guides