The general guideline is that your PS should not exceed 2 pages and it must be double spaced, 12pt font. But every school is different, so it boils down to what schools you're applying to is looking for.
Be careful with the modifications! After reading pages and pages of statements a small difference does stick out a lot. I've been doing 1.9 spacing, Garamond font (best thing ever), and 11 font.