Decide which type of reflecting telescope you wish to design.
Decide what the focal ratio or f-number of the prime mirror should be. This is the ratio of focal length to the diameter. Typically in the range f/4 to f/16, depending on the design.
Multiply the diameter by the focal ratio to calculate the focal length.
Radius of curvature is twice the focal length.
For a Newtonian telescope, you probably need a parabolic rather than a spherical mirror.
Optical telescopes have one or more convex lens like a magnifying glass. Compound telescopes have mirrors within them which can be concave. Radio telescopes have a reflecting and focusing surface which is concave and reflects to a focal point and area whereby an electronic receiver processes the image electronically through circuitry and computer programs. The radius of this radio scope is determined by its 'f' factor, as explained above by Alan Robinson, where little calculation is needed.