5/8 and halfback are similar, they're the main playmakers. They're the ones that have to see the whole play and get the ball to where it needs to be, they're the ones that do most of the kicking, and they also need to be able to run the ball if the opportunity rises. They're generally smaller, but there have been some exceptions. For example, Wally Lewis was a 5/8th, and I wouldn't exactly call him small. Lock is more of a defensive, mobile forward role. They're basically someone who has a big tackle count, but also can break through lines with speed. They have to be big-ish at minimum, it's only really the Hooker that can get away with being small in the forward line.
