how big do you want to go? my first flip was on a 4' tall jump with the lip around 60'
you want a good amount of pop off of it, so a jump that sends you up is better than one that sends you long. the bigger the jump you don't have to throw the flip as hard, but if you crash it will be on a bigger jump.
really, flips aren't as hard as you think they are, and once you get them dialed you can do them off almost anything.
I'd say build a smallish jump (5' tall -ish) with a soft landing, and just send 'er