couple of tips on backrolls:
Rotation speed depends on where your hands are, if you can take the rolls huge and you want to lay them out and rotate slow, then let your arms out, but for people just learning the trick and not quite making it around you need to have your hands close in to your waste as you leave the wake. Try to hold them in there throughout the whole rotation, it will be tuff, but this is what will help you make it all the way around and land on the board. If you are landing on the board but it is sliding out from under you, try taking it a little bigger (more aggressive cut) and letting your arms out as you finish the rotation, you can easily over rotate if it is big, the key is to have enough time to spot the landing about 3/4 the way around and then slow down the rotation and set the board down perfectly standing up straight-without having to butt-check the water.
Hope this helps