AB,
I ride a gxxr1000, and I don't feel a bigger bike is NECESSARILY more dangerous. However, that thing will go warp speed in the blink of an eye, so some discretion is advised when riding. As PDP said, you CAN die plenty quick on a 600. Any Japanese sportbike is wickedly fast.
If you can
test ride some that would be great. It has been my experience that it is difficult to find a dealer that is willing to do that (especially with sportbikes).
I think buying ANY bike used is a scary proposition. Cycletrader is probably no worse than the newspaper classifieds. If you choose the used route, try to find an individual(not some small time dealer) that is an adult. Plenty of "older" guys buy sportsbikes on a whim, then two years later they decide that putting 500 miles a year on the thing is not cost effective.
As far as which brand, I don't think will go wrong with ANY of the big 4.
Good luck.
Do you ride now? If not, you might want to consider a Motorcycle Safety Foundation course for some training. I know it sounds boring, but do it. If you ride a bike the way you drive a car, you WILL get killed or maimed.
That being said, there are few things sweeter than getting on the freeway and cracking the throttle open and 0-100 in about six seconds. Too cool!