I wouldn't make a bedroom smaller than 10' x 10'. IMO, it would just be too small. Think of the MINIMUM furniture you'd need. Take some tape and mark an area on your driveway that's 8' x 10' and then size up some furniture too. Look what room is left??! You need a closet too and room for doors to open and shut.
10' x 10' for a bedroom, 8' x 10' for a full bath (no shower stall.) YOu may want to see if you can get away with a 6' x 8' bath. (These are all FINISHED dimensions.)