Dude that sounds really short. I'm 5 9 175 lbs. I rode a custom 56 for years and now ride a custom 58 for my all around. I've been riding for 19 years (season passes for 15). The shortest board I ever road was a 53 (too short) If you go shorter (little under the chin) you gain in freestyle manuverability and turning at lower speed. If you go longer (mouth) you gain powder float and highspeed stability. Also consider if you are average weight for you how tall ya are. If so go bigger or shorter. Style and personal preference is also a big part of it.
Test the waist width/boot fit by mounting your bindings so your toes and heels are centered across it. Put your boots in the bindings and rock the board from side to side. Press it a little do depress the camber. Check at what angle your toes or heels drag. I ride as narrow as I can without getting toe/heal drag. Then you get fast side to side rocking. If it gets more than .5cm too wide the board starts to feel cluncky. It will float better though.