I used 11 pt font with my PS.
Also, while uniformity is preferable, if your PS is just not cutting it to fit within a school's parameters, I don't think it's unreasonable to change the font. I had to do this for my UCLA application since they specified 12 pt font for the PS.
ETA: Check the instructions for each of the schools you're applying to as some will give specific parameters for the formatting of the PS.