If you're referring to the L designation on head racquets I can assure you that what he uses is likely not even remotely similar to a stock L frame in stores.
I thought OP was referring to grip size. And if Radek truly used an L6 grip, the mystery of his success with the ladies on the pro tour would be revealed.
HEAD used Swing Length Long to order their frames from fast Long strokes L5, L6 to the more compact game improvement sticks.Wilson used si, stiffness index to also,inform customers.