Hi-Massilia completed with up to 13,000 population.
That is as far as I can push it, and even then this city will not remain stable for a long period of time.
The problem is that I took too long to progress and the aging labor force took its toll very soon. The city is short of nearly 1,000 employees!
In addition, most industrial raw materials are imported and will not support that many population (at least not with this layout, the distance of goods delivery will delay production and logistics)
You win by sending a gift to Caesar. You can afford a lavish one and it will suffice.
At least this port city is what Massilia "should" be like.