Take a Non stretchy string, wrap around the tube in question. mark where they cross, measure that length, divide by Pi, 3.1416..

Why? You planning on cutting? If you do, then find a piece that slips inside the frame tube about an inch long. Make line up real easy & make welding it back easier & stronger.

Well,, if no one pops the answer up, you could buy clamps that you think will do the job, you know its not huge & its not tiny, clamps only come in so many sizes & arent real pricy, so, get enough in different sizes to do the job & return the ones you dont use.. Ive had to do things that way several times,, Of course, that means you WILL make 2 trips to the store. You mite get lucky just looking at the clamps & grabbing the size you think will do the job,