Time machine 128gb usb for my macbook pro

Ive asked this before but just wanted to ask a slightly different question about it. Is it possible to use a 128gb memory stick for a time machine back drive for a 256gb macbook? What im asking really is "will it let you" use it or will it state its not large enough for it to work? i only use about 50gb max of hdd space on the macbook, everything is stored via external drives and the rest of the space is just free space and will never be used.