Get less than a 400 and you'll be trading it in by the end of the summer and you'll never be able to get your date on the back
Some mid displacement bikes are faster than others. Also consider weight and saddle height.
V-twins (cruisers) tend to be slower off the line than similarly sized inline-4s (sport bikes). Dual sprots tend to have high saddles and some folks can't get both feet down. Older bikes can be really heavy and unwieldy at parking lot speeds.
The attitude of the rider is gonna be a bigger safety factor (imop) than the size of the bike itself (of course, riders with good attitudes may tend to get smaller bikes anyway...)
Controlling the motorcycle isn't very hard to learn - it's anticipating the cagers and staying alert that takes effort.