If you need to be full time according to Graduate School standards, you can review the Graduate School Rules (PDF) (Section 8: Full-time status and Minimum Registration Requirements)
For financial aid purposes, students enrolled at least half-time (4 credit hours) in a graduate or professional program and who meet all of the other general eligibility requirements are able to apply for federal student aid.
If you need to be full time according to the standards of the Financial Aid Office, you’ll need to check with them. Financial Aid’s full time and part time standards are different than those set by the Graduate School, and are established based partly on federal law. If you have any financial aid, you MUST make sure you are in compliance with the registration requirements of their office.