2048 seems to be the maximum number of units in a game, regardless of civilization. The last unit before receiving the above prompt was unit #2045; I'm making the assumption that slots 2046 and 2047 are reserved for one Barbarian leader and one Barbarian combat unit, adding up to 2048.
(Ah yes, maximum city size is 127 (i.e., 128-1); hit 128 and ... well, I forget what happens, but it ain't good.)
Ye, when a city tries to grow beyond 127, it shrinks to minus something, either -1 or -128. I've never got a city this big without cheating, the largest city i've ever built was somewhere around 100, and that was on cheiftain. But i think there was a thread somewhere around here where someone got carried away with the food caravans and their capital imploded.