As far as sizing goes I use one of two methods:
1) the Ford, Chevy, Cadillac method, if there is a Ford in the driveway I size a 1-1/2 ton system, a Chevy gets a 2 ton system and a Cadillac gets a 3 ton system.
2) the finger method, I stand across the street and hold my hand about 6" in front...