The first question I would be asking is how comfortable are you on machines,, any machine,, and how smart is your throttle. If your on a snowmobile,, a motorcycle or a four wheeler,,, they can all kill you if you cannot comprehend accleration and speed. If you have a smart right wrist (or thumb),, you are careful about how much horse power you apply which will keep you alive.
Any bike size will kill you if you don't treat it properly,, a 250 enduro will crack you head open in a corner,,, you'll just be doing it at the bikes max speed,, but your still doing it.
Bottom line,,, if you want to ride a bike you need to be smart about it.
I've been on r6's and gsx750..... I would recommend the 750 because it has more torque and longer gears,,, I find it more forgiving,, I can cruise in 3rd gear and my brothers r6 is in 4th,,, I've been away from bikes for a while so I feel I'm kind of learning again,,, and I have no problem with the 750,,, just be smart.
my 2 cents worth..