Start your rectangle at 0-0 and drag it across the top until the first number hits the end (we'll use an example size of 350x100) which would be 350. Now drag it down until the second number hits 100.
600x200 is not a good size for a sig at all. I usually use 350x100 or 380x130.
Also, I need to know where you want your name and if you have any font in mind.