What size top case would be best for INT650, don't want too small OR too big. Need to fit one full face helmet and a little gear. Prefer Monolock with back pad. Thanks!
Welcome to the forum.
Don't know which country you live in but ... There are piles of choices and you will find the right one.
You might start by measuring your helmet Cubic volume listed for these boxes can be misleading if they are not tall enough and you don't want to lay your helmet on its side.
You should also decide how / where will you mount your box. If it will be on a standard Interceptor luggage rack... check out top boxes for scooters. They are the same but usually have a smaller "footprint" for small racks.
Let us know ..