The linedef length is shown when you draw or drag a line or vertex. When drawing, click the left mouse button when the proper length is reached. It couldn’t be any simpler!
When changing the length of existing lines, place the cursor on the object, press the right mouse button and move. Now just look at the screen till you see the length desired and let go of the mouse button. Pretty easy huh!
When you have snap-to-grid turned on, the length changes in increments set by the current grid size. If you want to set the length to any size, set the grid size to 1 or turn it off.