Is a research assistantship considered full-time employment? For something I'm applying to, I need to know if I'm considered a student, a part-time employee, or a full-time employee.

I am a graduate research assistant with a yearly stipend.  No other employment is allowed.  20 hours a week of research is expected according to the contract.
I don't have citable information to give you but being a doctoral student and usually being on a research assistantship, anecdotally, I can tell you that in the United States, if you are a full time MS/PhD student and are on a research assistantship then it is not considered a full time job. Your full time job is that of a "student".

This was confirmed by an HR representative of my university because I asked her this exact question.