Changing Dimensions/Measurements

How do I change a dimension to be the exact value I want, in real time? For instance, if I make a line drawing 20 inches wide, how can I change it to be 13.33 inches wide by typing it in? I know you can do this in Solidworks as well as other CAD programs, but haven't figured out how to do it in Draftsight. I can see the measurement (Tape Measure) icon and the measurement value in the preoperties window, but the program will not let me enter anything into that bar. Any help is appreciated!