To my knowledge, there is no specific formula for landing gear size. The main criteria for landing gear size is that it supports the weight of the plane. Secondary criteria are that the LG should be the proper width and properly located on the fuselage for good ground handling. It should also be tall enough to prevent prop strikes under normal landing conditions.

All that being said (and I get wordy at times), you should use the TLAR system (That Looks About Right).

Dr.1