If you size pellets its only in one direction, 'down', which will usually cause a loss in perceived power, due to this power being diverted to/lost in the reforming of the pellet to fit the barrel.
you need to accurately measure the internal dimensions of your barrel to find the correct size you need, otherwise your just wasting your time. I believe it is any way.
Just find a batch that shoot well, then select out the best, ... head damage is critical, tail damage....not so much.
Weight also plays a part, but you dont need to be anal, jut discard the overly light or heavy within the tin.