well, there is Roman census data IIRC that gives the population of Rome, though I don't know if it goes back that far. I think those can be found online for free.

as to Massalia itself? to my knowledge there have been no estimates of the population of Massalia prior to the 18th century (for those who don't know, Massalia = Marseilles), beyond the fact that it quickly hit 1,000 after its founding ~600 BC.