My 4'11" daughter ( average proportions) rides a 42 cm touring bike, our other 5'4" daughter ( long legs) rides a 52 cm touring bike , and my 5'3" wife ( average proportions) rides 47 and 50 cm cm road bikes. My wife's 47 cm bike has a custom built frame that fits her like a glove.

This might give you some idea of where you fit. However, all the bikes are road bikes, and have a different geometry than the Reid bikes. If you are at the top of the "S" scale, I think the small would work for you. It looks like the Cruiser has a lot of adjustment to keep you in an upright riding position. Then again, if you having short legs means you have a longer torso, the "M" size might also work for you. Is there an opportunity to try the different sized bikes out? That is the only way you will know for sure.

Good luck!