Step 1: A Simple Rectangle.
Step 2: Funky Dimension Names
Select the sketch (1) to edit so that the dimensions show. As you can see, Alibre automatically assigned them a name. Only D1 and D2 show because the thickness dimension, D3, was assigned by the program when we extruded it. Lets rename them.
Open the Equation Editor (the F(x) button on the right toolbar ). Change the Names in first column to WIDTH, LENGTH, and THICKNESS. Click OK and see how the dimension information on your sketch change.