No one knows. everything is speculation.
Actually, we have quite a good idea. Going from the Forerunner novels, the Origins shorts, and Halo CE: Anniversary, Forerunners have a complex, striated culture of various trade guilds, all with their own unique customs, traditions, and beliefs. The protagonist of Halo: Cryptum and Primordium, a member of the Builder class, first is described as looking almost identical to a human.
Many Forerunners go through "mutations", though, in which their bodies morph. Some can be three meters tall and very beautiful, if the transformation is a success, while the less fortunate are described as having shapeless masses of muscle and tendons for faces.