If there are no specific instructions, then arial or times new roman, size 10-12. Using this will always be correct! However, be very careful. Some schools have specific instructions on the superficial elements of the PS and essays - such as double spacing. If they do have these instructions, make sure you follow them. If you do not, it is an indication that you are unable to follow instructions, implying your inability to be a good student.
It does matter if there are specific requirements. I only wrote this because I have seen all sorts of silly fonts, colors, styles, etc on applications and resumes. Some people think this will get them attention; however your B-school application is not your chance to be Elle Woods in Legally Blonde. Your resume should not be pink or scented.