A lot more than just weight comes into play. Body type and bone structure can also come into play.
Barrel bodies like Sapp and Wilfork who are short but are able to carry the weight and still play with good power and burst of speed. Once in awhile you will see someone like Zach Thomas who was what 5'10? Total tackling machine.
In a Belichick system you need strength,height and weight to play OLB.
There are rare exceptions to the rule but normaly he wants his players a certain size and doesn't seem to be one to wait and see if player Z will possibly put on 10 or 15 pounds. Size seems to matter, but recognition,loyalty,intelligence and work ethic go a long way also. But the fact remains if you can't set the edge, tackle and cover equally well, you better be able to do something better than anyone else can like rush the passer or be a special teams demon to see any kind of time at all on this team.
I don't believe that there is a true prototypical size because they have varied over the years. But i would guess if you come in lower than 6'2 250 you won't be starting anytime soon in a Patriots uniform at OLB. Unless your Von Miller at 246.