My general opinion is that if it's a pre-90 spec model, then it should be small bumper, if it's post-90 then it should be big bumper.
But I've seen a pre-90 small bumper look bad and a post 90 big bumper look bad.
Some how when I look at a pre-89/90 car, it doesn't look right unless it has small bumpers.
It's up to you though, it's your car.
I like a clean euro/retro look small bumper car, or a phat, low, big bumper mk2, although my current taste is for the former.