So Im about to order an OFR to replace the Edge III in my ibanez, and it has an option on the site that says "choose the string nut size" and it has one measurement that is 1-5/8'' 10'' Radius, and that one doesnt cost, but any other measurement costs like an additional 35$. How do I know which one to choose?
The 1-5/8" is you average mut size. You can easily measure that with a standard 6"ruler. The radius can differ between brands of guitar. The only way to really measure that is get a tool to measure that. You also can get a drawing compas and draw a line at a 10 degree radius. Cut that out of a piece of paper and see if it matches up to the nut. It may not be exact but it will give you a close estimate of the radius.
Couldnt I always just choose that standard one and then not use the nut? Its not like the nut is causing any problems on my current setup, I could just not install the floyd one.
Ecactly. That nut is already set up for the specific guitar so keep it on there and if something happens to it you have a backup.