I know this probably isn't helpful, but it really does depend on the overal diaper pattern you are using.
Bigger soakers are more absorbent, but they are bulkier and take longer to dry (unless they are quick dry).
Smaller soakers are trimmer but less absorbant.
One thing people have done with PooPockets is make a pocket diaper so that you can stuff it with a soaker that is appropriate for your baby's size and age. You could do the same thing with a lay-in or snap-in soaker.
I have been making diapers with an hour-glass shaped soaker (snap-in), which is less bulky in the legs..but also more of a pain to sew than rectangular soakers...so I think I'm going back to the rectangles.
I think the best thing is to play around until you get a balance that you like. Diaper preferences are such a personal thing, IME.