For pretty much all of my SOPs (unless stated otherwise), I used Times New Roman, 12pt; Single-spaced 2 pgs max; Bolded title (Statement of Purpose; or whatever the program is calling it); my name centered and italicized underneath.
I’m a part of an NIH research fellowship aimed at sending students into PhD programs and this or pretty much their general recommendation for a clean SOP. Specifically, 2 single-spaced pages since double-spaced appears too long.
Good Luck