For enlarged prostate (benign prostatic hypertrophy), a range of 100-960 milligrams of saw palmetto per day is recommended.
Saw palmetto has quite a bit of human studies backing it. A dose of 320 milligrams of saw palmetto extract, in one dose or two divided doses (80-90% liposterolic content), by mouth daily for up to six months, has been taken in numerous studies.
The results though are mixed. Some show it helps, some show no positive effects.
Saw palmetto supplements are pretty cheap so you could give it a try, just don't expect miracles.