Okay, so if all your cavalry either has the shock special rule or the knight rule, which I believe combines shock within it, then you have less to worry about because they get mounted facing the short edge and should fit the length of the base. (The advantage of shock basing is twice as many stands may charge against a long edge based unit-- their combat bonus is built into the way the unit is based.)
Here is where it gets a bit complicated. I agree with Jchaos79, it's best to try to keep the standard 20x40mm basing-- it's part of the beauty of WmA where battle lines look like proper thin battle lines. However, if you are planning further armies, you may have to go with deeper bases to accommodate certain units at 20mm, due to not quite fitting on a normal WmA base.
Having played a couple dozen games using 15mm with DBx basing, I can't say using odd sized bases makes a huge difference in gameplay, yet you will want both armies to match in their troop type / base size.
As for figures per stand, WmA is mercifully accommodating-- most players simply put on as many figures per stand as they like, usually fewer to identify troops fighting in loose formation (skirmishers) and more to distinguish close order units.
I hope this makes sense. I fear I've made it more complex than it actually is.