Donald,
The pixel sizes listed in the Mesomap setup usually have to be tweaked to a larger size then the actual image file you are using in my experience.
The FAQ you mention:
http://discourse.weather-watch.com/t/9633
Doesn’t address that aspect as it doesn’t seem to be a constant. If your map is over 500 pixels in any direction I would recommend resizing it downward if you can’t get it set just the way you want as I believe there is an upper size limitation as far as displaying properly…
Try making a size change then force an update, wait a few minutes and then see what the result is on your web site. I had to tweak some of mine a number of times to get them where I was happy.
-Bob