Not at all sure you can with studio, I see no setting for it in the regular or advanced settings{DAZ's render settings are quite simplistic}. I am probaly wrong because I don't render in studio, for me studio is just a prep app for a real CG application, Carrara. Where DPI is a render setting {and where I found the fix for the fuzzies in carrara renders lol, setting it up to 96 really crispens a render}.

300 DPI sounds like print work, large posters and such.