A job application letter should be no more than one page, single-spaced with 1" margins in a traditional font like Times New Roman size 10-12. It should include your contact information at the top, the recipient's contact information for a written letter, a polite greeting, a 3 paragraph body discussing the position and your relevant experience, and a closing compliment with your signature. For an email letter, include your contact information after the signature line instead of at the top.