3/4"
MrCuervoRacing makes an early-on fan thermostat, turns fan on at 200 degrees f, and mounts in the upper hose.
I have one and it works very well.
The fan should not be coming on that often as long as you're moving, unless it's a very hot day.
I have an inline cooler in the lower coolant hose, Engine Ice coolant, Spal fan(440cfm), Mishimoto radiator(30% larger), air scoops, Vapor dash with 2 temperature lights, and a bigger aluminum oil tank that holds another quart.
These are all the mods you can do to the cooling system, but they work great.
My Vapor dash has memory, and the hottest my 700 has been is 217f, ideling after riding hard on a mid 80's high humidity day, I watched it move up slowly from 211f when I stopped.
Pretty big improvement considering the stock fan doesn't turn on until 225...and
I didn't even have the oversize radiator then.
The cooler it runs, the longer it'll live.