Yeah, 250 would suck, I've had my 650 twin for around 3 weeks now and it's starting to feel like a moped.
Another thing is that a 650 twin is that it's probably the fastest your going to get without having to worry about pulling the front wheel up. The only times I've pulled mine up is hard shifting into second and an intentional clutch-up.
Well that could also depend on the gearing, my 525 can easily do wheelie up to gear 4, it was actually difficult not to do wheelies when i started on that bike, but thats probably due to its lightweight.