Typesetting statement of purpose for a masters application
You should follow the instructions. Absent any such guidelines, you can do whatever you like, within reason. In the US, "reasonable" tends to mean:


Times New Roman, Arial, Calibri, or something similar
Single spaced, or something close to that (e.g., 1.1)
1" margins, almost always
Left or justified alignment, personal preference
Indent your paragraphs or leave a space between paragraphs or both
Header on each page including your name, page number, and any other relevant information you'd like to include. (e.g., university name, application number)


Personally, I like to use letterhead (either institutional or with my name on it) for official documents like this.