"The largest I've built, with over 2 years on SC4U, is only about 175,000. My cities seem to stall whenever I get over 100,000, and even if I build industrial suburbs to try and get the city to grow commute times go up and buildings start to abandon."
Transit my man, transit. With a good transit system I was able to actually limit all the high density comercial zoning to one small city. where the region actually extended out to the limits of the map.
Lots of highways/avenues, bus stops in the low income parts of town, mostly connected to my industrial sectors
Once you get past 100k residents you have to start catering to the middle and upper class. schools, parks etc. it will increase the likeliness they show up and it will increase the demand for your manufacturing and higher level comercial buildings.
trains are handing but a pain. or so Ive found. Ive never quite got a good fix on how to zone it out. Ive found it isnt really needed as the region I was refering to earlier had a population of approx 4 million, and I only connected the most densly populated cities with rail. hope that helps you get over the hump