It's all in one of the FAQ's, but one of the most useful EU formulas is for the cannons required for a seige:
3 x the level of the fortress.
So a 'Small' Level 2 fortress requires 60 cannons. I usually send in more than 60, up to 70 say, to allow for attrition, which is small for cannons, but taking in 60 and losing one to 59 lengthens the seige a lot.
If you have this nummber of cannons or more, you get level 3 (the number in the left hand circle on the seige display). The higher this number is, the faster the seige goes. The only way of getting it above 3 is to take a leader who has a seige value which is added (so leader seige value of 2, plus say 65 cannons on a Small fortress = level 5)
So far as I know, and in my experience, more than this number of cannons gives no help and just adds to attrition problems.
Level 6 is the biggest fort, so you never need more than 180 cannons for maximum seige strength.