I used this plug:
Sick Bike Parts NGK B5HS
and this wire:
Sick Bike Parts High Performance Plug Wire
You have to change the wire when you change the plug though. The old wire does not work with the new plug and vise-versa. Just order them both at the same time. The new plug wire is extra long and you just cut it to length.
I simply measured the length of the plugs from the tip of the electrode to the shoulder of the threads to make sure they were the same length, and they were. I would feel pretty safe in saying that this plug isn't too long, but ultimately it is your responsibility to make sure that it won't hit the piston. Be sure to gap it.
It is a really bad
thing to run the motor with a loose spark plug. Chances are that it was vibrating loose (one of the items that should be in the pre-ride inspection).