First, even if the forum resizes your picture, it is still good to try and get it close. People on modems still have to wait for your large file to load even if it appears small.
Look at the par-metal website. They have all those dimentions for their products. We don't know where you are, so you may not get it from them, and every company may be a little different.
From the edge of the panel, to where it meets the enclosure, it's 1". In other words, the panel should be 19" wide, and the box should be 17" wide.
The mounting screws' position is different for every enclosure. Every enclosure manufacturer should have dimensional drawings online, or they'll probably fax them to you...