I would call the "fully developed" Dunsmuir Roundhouse about a 1/3 circle, maybe even 4/10s of a circle. The last built three or four stalls on the south end, long stalls for the "Dunsmuir Mallets" lasted until the big snow of Dec '66 (Or thereabouts, I was pretty young!) and was used, at that time, as the Dunsmuir Corporate Shop building (where the metal shop building now stands). The Mallet Shed (could handle a cab-forward), still standing (Doors from the former Colusa/Gerber engine house) and the access road prevented the roundhouse from being a full half-circle. At the other end was the powerhouse and chimney.
Wish I had more than faint memories of the part of the roundhouse, one didn't take photos everyday back then! (Brownie box camera, and I never quite got the hang of using it without moving it a bit while triggering the shutter!--lotsa a blurry photos back then!)