Steph, as Randy's reply indicated, it's actually more important to have the floor(and/or ceiling)acoustically "soft". Some authorities(including Toole and Moulton)point out that some lateral early reflections from the side walls are beneficial from the standpoints of imaging and spaciousness, while vertical reflections don't seem to share this effect.
As to your room dimensions, the basic idea is to avoid having them equal or even multiples of each other. So, your proposed length and width are in excellent proportion. The ceiling height should also be considered.
