On the unit size qu, its up to you really. I find that default keeps the units small enough so there is some fun manoeuvreing on the battlefield, even if the maximum number of troops you can have on the field at once (16 units of 100, and chances are you will have more like 1000 as most units are less than 100 men) is a bit small. Its more like a medium sized skirmish when you think about it but it needn't spoil the fun.

Personally with bigger unit sizes I find the game less tactical but maybe thats just me.

The most fun battles tend to be in the early stages IMHO. Making the best of, say, three units of spears, some archers and a few light cavalry gives you a chance to make a difference to the outcome as a general. Throwing an army of say 6 upgraded Longbowmen, 6 billmen and four chiv knights at some poor saps in the later stages has its appeal too, but you'd have to try quite hard to lose. I guess that's what Expert is for. Don't feel bad about using autoresolve to get out of battles that look boring.

Oh and on my fitst game I made the mistake of thinking that titles were sweeties to be handed out to keep generals loyal, and tended to keep a load unassigned in case my generals started rebelling (which is not in fact a big problem). Provinces without governors make a lot less money than ones with even mid-acumen governors so always assign them right away.