I always fight with two types of armies: Battle Armies and Assault Armies.
Battle Armies:
Consist of up to 60% Cavalry with the rest being infantry only. I try to always keep them at at least 25% cavalry, which is usually not hard. Typical size are between 15,000 - 25,000 men; very rarely larger than this, as the army would then suffer too much from attrition.
I use these armies to attack enemy armies that have just assaulted any of my cities (i.e., meaning they are weak), to put down rebellions, and to defend my Assault Armies, if necesarry. Occasionally they are used on raids to try and kill of enemy cannons. An important point is that I NEVER use these armies for sieges if I can avoid it, and I ALWAYS withdraw them to home territory whenever possible (which is most of the time since these armies are usually only busy early in the war). Cavalry is far too expensive to waste on attrition.
Assault Armies:
Varies depending on the period. In the start, I usually go for armies of 8,000-12,000 infantry with a number of cannons depending on the fortress period; minimal = 40 cannon, small = 60 cannon, medium = 80 cannon, etc. I have found these sizes quite efficient at sieges and as long as they are kept around the size 15 mark, attrition is usually very low (or nonexistent) except during winter.
I usually tend to have 3 assault armies for each battle army in my forces - the idea of warfare is to win (i.e., take the other guys provinces) - not waste men and horses fighting his armies.
IMO, this is the best configuration for warfare. In a 1 on 1 fight, I can smash any AI nation in 8-16 months using such a configuration of armies (a typical strategy if all 4 armies are at full strength - besiege 2 provinces you want and their capital; when those 3 provinces fall - as they will relatively quickly - you then have 6 stars and can ask for those 2 provinces + 250d in the peace).
Pure Artillery Armies: Never create them. Never even recruit artillery alone. You always need infantry to take the attrition losses.
Pure Cavalry Armies: The same. Cavalry die far too readily in battle if there is no infantry with them, so pure cavalry armies are pure folly. Only place they make sense is if you need to send some fast reinforcements to a beleaguered army.
Pure Infantry Armies: Never use them for anything important on their own (except island garrisons), but I usually have hordes of 2000-5000 man regiments running around - reinforcements for all the siege armies in enemy territory.
Those are my 250d...