Hey,
I find that if you go straight from 2 wheels you don't have as much momentum to jump as high. So I can go bigger going off the back wheel.. I start off getting my balance on both wheels parallel to the ledge I'm aiming for, once your ready, I do a quick back hop to get me on the back wheel then quickly let my front end drop (almost to the floor again if I'm going really big), crouch as low as I can over the back wheel stretching my arms slightly ready to jump. As You jump and kick, release the back brake slightly as you do, and jump as high as possible pulling the bars towards you and leaning to the side. Once your front tyre is over the ledge, push the bars forward holding your front bake while bending your knees so the back wheel can then follow your bum up, and hopefully onto the ledge as well.. Once you've got that and you wanna go even bigger, you'll start needing to put your back wheel to the side of your bum once your on top of the ledge so it doesn't get in the way. Otherwise you'll be wanting to go higher than your back wheel will let you, and it'll end up hitting your bum, and you'll probably fall off because it won't lift high enough..
Bit of a rant, but hope it helps!
Rick