Typical lengths are both 36 and 48 inches.

If you are doing a sheet side, the 45 degree angle will work fine, but it would be better as a lap joint. A lap joint means that you glue another thin sheet of balsa right behide the joint. 3/4" to 1" will give plenty of glue surface, with the grain crossing the joint line.

If you are doing a lap joint on a stringer or spar (especially the spar), then you want the length of the joined surface to be 10 to 12 times the thickness of the spar. Be sure to line up the grain of the wood with the cut.