I have a few air planes and want to get the right spinners for them, How do you figure out the right size?

Size is sometimes dictated by the cowl. If your plane has a cowl, measure the front and get a spinner which is that approximate size.

If you have no cowl, the choice is yours - there is no right or wrong size.

But note, that all spinners are usually molded for the smallest size prop that would be feasible to use with that spinner, so in most cases, you need to enlarge the holes for the prop.

Get whatever looks good and/or fits the lines of the plane.

I make templates out of paper; you can go to a website and save the picture of a spinner, then print it and resize it if needed. I hold it up in front of the plane and see how it looks, and it works pretty well. I also sometimes just cut them out of paper. Look at the dimensions of the spinner - if it is 3" spinner and 3 1/8" long, you can pretty much fudge the shape and make it look good enough to decide on the size.

Hope this helps.