Most of the DIY cameras being discussed here are the folding field camera type (unless I'm mistaken). Most commonly, these use 5" x 4" or 10" x 8" sheets of film.
This type of camera is also normally used with a lens with integral shutter but they can also be used with a pin hole (no lens) or barrel lenses without shutters where some ingenuity is required to control the light.
EDIT: Actually, I assumed incorrectly. There is a good mix of all sorts of formats in this thread.
For some ideas, have a look through the 'workshop' section here: http://www.raymentkirbycameras.co.uk/