For online delivery, I use the HD 1280x720 format, simply to save render time. In fact, there are times that render a scaled down version and then up-rez it in the video editor to soften the look a bit and decrease the Carrara (or any other software) rendering time. But only if there isn't crisp detail needed such as text.

Yes, more pixels means better resolution but time is the trade off.

My broadcast stuff is sent to a render farm since they're now consistently requesting 2K and 4K output. Not enough hours in the day for that rendering in my studio.