A 42mm radius is the standard, same as the finger-board. All my fiddles have a 60mm radius . . . which happens to be the same as a CD.
If you do this, make sure that your D and A strings dont get too close to the finger-board and buzz. Since your G and E strings will be set at their usual heights (whatever that is) off the board, your D and A will now be closer to the board.