I have a tendency to make cities on the larger side so I was wondering what everyone thought the best cultures for such cities are ? I was thinking maybe Olmecs and Celts early since those bonuses seem like they'd add up over time ( particularly the Nemetons +3 food per territory bonus )  but I honestly have no idea 

Large meaning many districts and / or many territories attached, I assume. You would need a lot of influence to attach consecutive territories, and a lot ot stability to build districts. I recommend Austo-Hungarians. I chose them exactly for the above needs, and had no stability issues till the end. Ended up with 11 cities, each 4-5 territories, making 5000+ production, 3000+ gold and 2000+ science.

Agree - you're going to need influence at first. The population will grow naturally, especially if you're in good locations within the province. (ie you don't need a ton of districts).

If you don't feel like maximizing your influence, then go for gold. There's a civic that switches the need for influence to gold when attaching/building outposts. It's much, much cheaper. So I'm doing a full merchant run (Nubian, Ghana, Carthage, Dutch, Khmer, Siam) - and have 3 cities with ~10-15 territories attached and a few minor cities. No stability issues because of all the trade routes w/patronage.

Key is Ming... the only way to get the influence to be attaching your 25th territory in a reasonable time. (although I Guess Inherited land Venice+Siam+China is another option)

Any culture with an emblematic district that have a scaling effect based on number of districts or maybe population is a strong choice for a huge city. I take Japanese as an example as their robotic factory give +2 industry on each maker quarter and +2 science on each research quarter. Now if you have a city with 30 territories and you build 30 robotic factories, your makers and research quarters in that city will get +60 industry/science and keep in mind that the robotic factory is both a makers and research quarter, meaning it get +60 industry and +60 research and with 30 of those that is 1800 of each resource. And now lets assume you have 100 makers and 100 research quarters, that mean an additional 6000 industry and science for a total of 7800 of each resource and with even bigger cities and more districts, those numbers can get much higher.