I just built a 21 inch SL Octave Mandolin (GDAE, 1 octave below a Mandolin) and I found the string sizes on blogs. I ended up using the strings from a set of Fender 150XL/012. .053E string tuned up G; .042D D; .024G up to A; and .012E E. DAE are good, but the G vibrates and flaps around like a happy dogs tail. So the question is....Do I go up from the .053 string size, maybe to .058 or down to maybe .048?
Try this page from Ry Cooder. The string gauges shown are for 23" scale, but they are close to yours, and give a good source of info.
Thanks Scott, I'll try the .042 for the G string.
I tried the .042 as a G, but it put too much tension on the neck and loosened the other strings, so I put on a .048 and that did the trick, so I now have 12-24-42-48 for EADG. Thanks! I'll post a video....someday.
I have a solid ody electric tenor that I strung and tuned to GDAE. I used a set of electric D'Addario EXL110-10-46's, using the .013, .017, .026, .036 strings. Tension is very comfortable.