OK so with your advice in mind I realized I was just confusing myself and came up with this simple solution:
My radial menu desired size is 256, and the distance from the center to the outer edge is 122.5, so 256 / 122.5 is 2.089, and just divide the desired size by that 2.089, and then multiply the result by the viewport scale factor, and that gives me the measurable screen distance! Yay.