Isn't this easy to do in MSPaint? Why do you need to get others to draw a rectangle for you??
I'm confused.. If the ratio is 3:2 then it would be easy enough to draw quite accurately on paper as well. All you need to do is pick a random straight object, use it's length for the 20m side then use it's length and half again (should be able to measure this accurately enough by eye) for the 30m side.
Unless I'm missing something painfully obvious of course.
Now if you were a true nerd you would look up the length of your graphics card, open your PC then remove the graphics card and use that as a reference of length.