As France I would usually keep a smallish army in the southwest to discourage Portugal from attacking. Having an alliance with them was always a top priority of mine - that, and securing the border with strong castles at Bordeaux and Toulouse.

That usually leaves 2 standing armies, both full strength - one in the northeast (Metz) to ward off Danish or German invasions and one in the southeast (Marseille) to defend against the Milanese. These armies would have 4 knights, 6 mounted seargents, 5 spearmen and 4 archers or crossbows. The cavalry superiority works very well in disturbing the enemy formations, who can then be picked of by concentrated charges against isolated units. Whatever reaches the main battle line will be decimated by archer fire, fixed by the spears and charged to death from behind. Vive la horsies!

One problem with that setup is the lack of a castle in southeast France - I could never take Bern and so it takes a long time to reinforce the cavalry near Marseille.