Cinema 4D defaults to using Centimeters as the scene units, however it is possible to use other measurements as well!
Before we get started, a little quick tip: Any field in Cinema 4D can do math for you, right in the field! For example, if you need a Cube to be exactly 2.75 inches, by 3.75 inches, by 8 feet (a standard 2x4 construction board size in the United States), you can actually type this directly into the Cube's size fields in it's Attributes:
And Cinema 4D will automatically do the conversion for you when you hit Enter, or click out of the field:
Cinema 4D fields can do a LOT more math as well, and all fields that can have a numerical value can do this. You can even enter equations and it will automatically display the result; but that's another article.
If you'd rather not simply convert the values to the default scene units (IE Centimeters), you can also change the scene units, and the Project Scale.
The scene units can be set in the Preferences under Units:
However, this will not change the Project Scale, which will likely still be set to Centimeters.
For example, if you set the units to Meters instead of Centimeters, the Project Scale will still be in Centimeters. If you were then to create a Cube, it would default to 2 x 2 x 2 m because the default size of a cube is 200 project units, which is this case is Centimeters; however it's displayed in Meters, and 200 cm is 2 m.
So, to display everything in the proper Project Scale as well (IE if you want a cube to be 200/200/200 Meters instead), you'll also need to hit CTRL + D (Windows) or CMD + D (Mac) on your keyboard to bring up the Project Settings, and change the Project Scale to the same thing you set in the Units window in Preferences:
Once you do this your scene will be fully converted to your new unit of measurement!