City Life

Taking example numbers from the article, but not explicitly stated, it kind of sounds like 15 floors, about 60m tall, is the point in which density starts to sacrifice ecological concerns too much.

According to a 2015 study commissioned by the CTBUH, the whole life emissions of both energy use and materials for a 120m concrete and steel structure are nearly five times higher than those of its 60m equivalent.

That's still tall enough for many things, so I guess the rest should be handled by a bit more space use and better mass transit.