You should be provided with a WPS that provides the welder with the information needed to fit and weld the test coupon. Oops, wait, you are probably welding to ASME in which case the WPS will most likely tell you everything except what you need to know.
Groove angle: 75 degrees
Root Face = Root opening = Electrode Diameter
Just a starting point. You can make adjustments to suit your preferences
Good luck -Al