Even in the physical body, intelligence is not necessarily tied up with brain size. Look at Cephalopods for example, which are highly intelligent yet have relatively small brains (although still of vertebarte brain bodyweight ratio); or insects, which seem to possess a sort of communal or hive mind. However, as far as vertebrate animals go, the proportion of brain weight to body size can be used as an approximate indicator.
diagram by Harry J. Jerison.
diagram from Paleoneurology page The page had an interesting article on
the fossil brain (endocast) of the Eocene mammal Bathygenys reevesi
Before one gets too cocky thinking of man as having such a big brain, and therefore supposedly being so smart, it should be pointed out that dolphins have a larger brain to body-weight than humans, and neanderthal man had a larger ratio than modern man.