When you create a surface, it automatically takes dimensions from the established area. You can override the area dimensions by opening the surface and editing the length, width, and height.

However, if you would like to input a direct square footage or linear footage amount, you can click the switch button.

To do this, open the area, select the surface you want to modify, and you'll see the dimensions displayed in the length, width, and height boxes. If you prefer to adjust the overall square footage (sqft) or linear footage (lnft) for the surface, click the switch to sqft/lnft button, as shown here:

By doing this, you can customize the sqft or lnft for this particular surface without affecting the measurements of other surfaces in the area!