I wouldnt think that size is really a issue in this sport, big = strong, but usually means slow too, not allways but mostly.
Small = not quite as strong, but bloody quick, well mostly quick
Its hard to make a 5ft guy submit if you cant hold onto him.
I think the basic idea, no matter the size is learning lots of grapples and take downs, couple that with lots and lots of weight training, and you should be great at this one.