When do you switch sizes on spinners like say there is 2, 2 1\4 2 1\2 2 3\4 3 inch and so on what size spinners go with what engines i have a 55 ax and a 75 ax what spinners whould each take and why would i use different sizes on the two?

Use the spinner that looks right on the plane. Doesn't have much to do with the engine.

I have a 2" spinner on my Venus with a 55AX, I had a 2.25 inch spinner on a 46AX on my trainer, and I have a 2.5 inch spinner on my 120AX and a 2.75 inch spinner on my 110FZ.

Sometimes you have to widen the slots to get the prop to fit, or you can get spinners with larger cuts. I got the "120 cut" for my 110 four stroke which fits larger props.

yes spinners you choose by the plane.

any good secrets on the best way to trim spinners without getting them too bad out of balance or does it matter

I have found that hand files work the best. I tried quite a few attachments to my dremel and a mini hacksaw, but nothing works as quickly or smoothly as a file. You can draw lines where you want to file down to with a fine point sharpie for some guidance.

As far as balance, that tends to be a personal preference in this forum. Some say the weight is too close to the axis of rotation to make much of a difference, others say everything spinning at 10-16000 rpm should be balanced as best as possible.

I bought a Tru Turn spinner because they either come cut for a large prop, or they will custom cut for you for a minimal charge, and they will send it to you balanced. I don't have the means to balance spinners otherwise I would balance all of mine.