Or also you can draw in a new rectangle, while you are drawing the rectangle it will show width and height of the one you are currently drawing, if you use rectangle 3 point you can snap the points onto the rectangle you want to measure and look at the width and height for the current drawn rectangle, and push Escape to cancel that command before placing the last point.

If you want to do it this way you may want to turn up the display precision under Options > General > Decimal display, by default the display will be rounded to 2 decimal places.

- Michael