Fab room ceiling height?

If going with an I beam/trolley, I'd keep it up by the rafters, above the bay and loft. If you want to also access the "fab room", consider putting a ceiling/floor scuttle above the fab room, so you can open it up to use the trolley to lower things from above down into the fab room. Make the entry door(s) to the fab room large enough to accommodate an engine stand, consider a double door.