How do you know what size spinner to get? I picked up several spinners at the Perry GA swap, but none of them fit with my prop (even though they appear to be the "right" diameter)

Thanks</p>

Typically you choose a spinner diameter that fits the front of the cowl on a cowled plane.
Also it used to be a performance "rule" that you don't exceed 25% of the prop diameter for the spinner diameter. Otherwise you would be covering-up the part of the prop that makes the power. For example on a 10" prop you would want a 2.5" or less diameter spinner.
As far as fitting the spinner cap/top to the prop, it depends what type of prop you are using. Some props fit in the stock cut-outs while others need the cuts-outs enlarged to fit around the blades.

With a cowl, the spinner size needs to flow into the lines of the cowl. Without a cowl, use whatever looks good to you.
Since a certain size spinner may be used with a little prop or a big prop, they make the slots pretty small and you may need to open them up.

Other then ordering a pre cut spinner for the size prop I'm going to use I have never had a spinner I didn't have to open the slots myself. Takes about 15 or 20 minutes with a moto tool.

Spinners are cut to fit the smallest possible prop; or not even that. You have to open out the holes to fit your prop.</p>

You can get big bird spinners. In other words same diameter spinner but the slots are cut bigger for the prop

Don't worry about spinner size versus the prop size. As long as you have at least 2/3 of the propeller exposed, you'll be OK. The inner third of the prop is mostly there for the ride, and to support the outer two-thirds, which do the actual work.