I know I am just one PD here - but I would say keep it under a page if at all possible.
Rarely, applicants will appropriately write very long essays - any absence during medical school, any issues of substance abuse, etc.
Otherwise, if it can't be said in a page, chances are, I'm not going to read it all. I would much prefer to read a 3/4 page, succinctly well written essay than a full page one that is padded to make it a full page.