The typical shape of the theater was circular based upon the design of the Roman amphitheatres within an octagonal structure, having 8-24 sides. The theater consisted of a pit, an open arena. At one end of the pit was a raised stage, projecting nearly halfway into the pit, surrounded by three tiers of roofed galleries with balconies overlooking the back stage. The stage dimensions were usually 20 feet wide by fifteen feet deep but could be larger. All stages had trap doors so ghosts could enter and exit. The theater could house 1500 people or more, including the groundlings who paid the least admission and stood during the performances. See links for more information and a picture!
Before the Globe Theater was constructed, dramas were often performed in patrons' courtyards; therefore a circular shape and the use of balconies in Shakespeare's "Wooden O" is a natural extension of this model.