I try to make my king size quilts about 120 x 120". But some LAQ have difficulty with that size. The wide backs are usually 106-108", so at 103" you barely have enough backing to extend out under the top. If you make it any larger, you'll have to add onto the backing, too.
King size beds are around 80x80", so for a 15" drop, you'd want it 110" wide. I'd make it 110" long to have room to cover pillows. Or you could leave the length at 103" and use a wide backing sideways.