Most of the time I'm aiming for big, high-density RCI zones but sometimes I want to build smaller areas that can't blossom into huge pollution spewing factories. Can I limit the density with clever use of space or roads?
In addition to the density of the road, as mentioned by Raven Dreamer, the density of a zone is also restricted by space.
This means that if you build your roads too close together, the maximum density of the zone will be limited, resulting in a 'not enough room to expand' message when you bring up the information for a particular zone.
I especially noticed this when building circular roads, as getting them too close to each other will prevent the density of the zone from increasing due to lack of space.
There is one more way to stop the growth, in addition to those already mentioned. In this iteration of SimCity, de-zoning tool does not destroy the building, but prevents a new one appearing in its place, whether it happens after demolition, or as a result of automatic upgrade.
As can be seen from the above screen shot,
Removing zoning from under buildings will stop them from increasing in density.