Do you need to use the RECTANGLE command to draw one or more rectangles that makes the closed polyline a set width?  Most people draw the rectangles and then use a polyline editing tool on them to change the width. Here's the easy way.

Start the RECTANGLE command and use the Width option by entering W.  Specify your line width, hit ENTER, and then start your rectangle.  Instant width and best of all, the next time you use the RECTANGLE command in the drawing session you will maintain the same width.