the answer on length is it depends on your chamber. Sinclair Chamber Length Gage - Sinclair Intl
Sinclair makes a small tool that will allow you to measure the exact length and and you can then make an informed decision on trim length for that chamber.
As for the round sliding around, IF you are talking about the bullet sliding up and down in the case neck, that means you do not have enough neck tension and are not sizing the neck down enough. If you are using a standard FL die something is seriously out of whack for that to happen. If a bushing die, you have too large a bushing.
That could happen if you neck turned the brass and tried to size with stanard FL die. When neck turning, you need to use bushing dies.