The conversion factor is around 1.18 for square to 12 pt.
In your case a 1 1/2" 12 pt socket/box wrench should fit.
Socket size should be smaller if it is flanke drive or spline type - same if the squares corners are rounded.
1 1/2"socket regular

1 1/2" socket for 1 1/4" square
Ratchet square drive examples:
11 mm or 7/16" 12 pt will fit a 3/8" square
15 mm 12 pt fits 1/2" square
22mm or 7/8" fits 3/4" square
You need a 19/64" 12 pt to fit a 1/4" square - I don't have a 19/64" 12pt wrench in my tool box to show
Note: 12 pt is not really the right tool for a square since it only engages the corners of the square - not for high torque applications.