Yes, the alto is smaller. They both have crooks in the neck just below the mouth piece somewhat like a sax. The Bb is heavy and quite a bit longer than an alto and rests on the ground between your feet during concert performances with the help of a stem stand. The alto is played along side the right leg.
Since bass instruments are generally bigger than the original instruments, I would say yes. The bass clarinet is tall and big, and sort of shaped like a saxophone. The alto clarinet is the same thing, but smaller. And the regular Bb soprano clarinet is straight and usually plastic. They all use reeds (bigger the instrument, bigger the reed). There is plenty of information.