Precursors have never even been stated to really exist; the Forerunners were the only ones who actually believed that they were real. Humanity nor the Covenant was never known to find a Precursor artifact or anything that pointed to their existance, except Forerunner records. No one knows what they would have looked like, as we don't even know if they existed or not.
Uh, we DO know that they existed (read Halo: Cryptum). See this article for what we do know, including a limited description: http://halo.wikia.com/wiki/Precursors
actually in halo cryptum, one is described as being vey large, having an arthropod-like body with 4 clawed upper limbs and two legs and also have flat heads with a scorpion like tail that comes out the back of the skull. but as the forerunners could modify thier appearence in halo cryptum and the precursors had even more advanced technology, perhaps the precursors could do the same, they could look like anything.
FIRST PERSON - Oh, really? Sorry guys, I didn't know; I never read Cryptum. I was gonna buy it, but it was pretty expensive so I bought Contact Harvest instead. My bad... Last I heard, the Precursors we just a sort of myth. Sorry if I mislead the asker.