Full siblings are the best way to lock in the best and worst traits since they're so similar already most times. Parent to offspring or somethi bff similar is really the closet you want to go unless you know what you're doing and don't mind having to cull heavily when bad traits start popping upInteresting, so it will just take longer breeding back to offspring to get the size is what I'm assuming.
Also want to make sure that breeding siblings isn't going to be a problem? I think I know the answer but had to double check.