Mate, I don't think there
IS a common size.
The ZZR600 is 17mm one side (the threaded side) and 14mm the other. I'm pretty sure once you poll the Kawasaki community you'll get an equally random assortment of sizes.
It's nearly worth going to a decent toolshop and ordering the one size you need, otherwise you're going to end up with a half-dozen large unused inhex sockets

.
I've tried looking online for a 17mm inhex socket and they're not at all common, although I can get one in a set of six from Sidchrome. I'm baulking at paying $70-odd for essentially one socket though.
It may be easier to get a 17mm allen key and get someone to cut the end off and use it with a 17mm normal socket. Alas they're hardened and the cut method may invalidate the hardness...