How brief should a "brief" cover letter be?

There's a job I'm applying to that says to send a "brief cover letter explaining my interest and qualifications." Given that a normal cover letter doesn't exceed a page, what's "brief" in this context? Half a page? 300 words? A paragraph? Help me out.

usa jobs
usa jobs resume
usa hotel jobs
usajobs
usa jobs federal government
usa job in ksa
usa jobs
usa jobs login
usa jobs gov
usajobs.gov
www.usajobs.gov
usajobs.com
usajobs
usajobs.gov official