if you're "on level" (hieght wise) with the creature I'd say it's just a grab, regardless of size, you're just grabbing on to it's shin or what not.
Alternatively, use your move action (prior to the grab) to do a vertical jump (see Athletics skill) to get some height and then do a grab as the standard action to get the arm or back. For the unexpectedness of grabbing higher i'd give a slight bonus to the grab check (after all, for passing the athletics check first, there should be some benefit; failure would have them land prone on any square adjacent to the target).
If the PC is already higher than the target creature (say on a rooftop trying to grab on to a giant walking on the street next to him) I'd treat it as a charge (ie. you're falling that extra distance; failure means you're prone adjacent to the target)...
and being on the creature should provide you some benefit (either in the force of defense from that creature's attacks, or benefit to injure it, else, why bother).
i'm sure there are some combinations i'm not accounting for , but that seems simple enough for a framework...