You can enter dimensions when you first create your objects. When you create a primitive, just click on the little box next to the menu item and a dialog box will pop up allowing you to specify the height, width, and length, etc. For example, I can enter 12 units for height and my box will be 12 units for height. It's up to me what 12 units is, whether I pretend they are inches, feet, meters, or whatever.

You can also go into preferences to actually define what the units are (inches, feet, etc.)

Edit: You can also create a box or plane a certain size to use as a reference when editing another item like the short wall. I'll post a pic to explain better.