A dimension is a version of a room that has limitations on population capacity. Dimensions are used to help members with similar preferences find each other, and to set restrictions on what type of conduct and appearance is allowed in a room.
You can switch dimensions by clicking on the Dimensions Icon on the Icon Bar at the top of your Curio screen.
This will open up the Dimensions box which will allow you to view the amount of different Rooms, the population capacity for each, and allow you to enter them.
Tip: If you find yourself in a region and can't see many people, check the Dimensions to see if there are more populous versions you can enter.