With any adjustable BC Jacket or Back inflation.
1) Start with the shoulders loosened up all of the way.
2) Tighten the cumberbund/waist strap, so that it is below your ribcage. This becomes the anchor that is going to keep the BC from riding up. Most BC's have a way to tighten the cumberbund in the back, so don't let this be the deciding factor on whether the BC is to big. You should be more concered with the torso length.
3) Tighten up the shoulder straps. Ideally they should end up about halfway down the length of webbing. This gives you some adjustment for different thickness wet/dry suits. If the waist strap/cumberbound rides up when you tighten the shoulders, make it a little tighter. It should stay in place below your ribs.
If the shoulder tighten all of the way and the the BC is still loose it too big. If they are are at the top of the webbing with no more strap to let out it is too small.
Finnally if the BC has a second strap over the cumberbound, conect it but leave it a little bit loose, becasue this strap ussually get tighter when the BC is inflated.
The final test is to actually dive with the BC and see how it feels with a tank on in the water. Make sure you buy the BC from a shop that will allow you to try a different size if the one you buy is not just right.
Paul