I’m not sure i can explain it right now…little fuzzy.
You can place the file anywhere you like on the board, but use the full 20" x 12" artboard. There is a workaround by Glowforge that corrects for resizing issues between design programs, but the ratio on the artboard needs to be that size. (The size of the bed.)
If not - Adobe Illustrator (which I believe Dan used to create the original file) exports SVGs at 72 DPI, and several other programs (like Inkscape) import at 90 or 96 DPI, depending on the version you have installed. (For instance…96/72 =1.33333.) So things get resized between the programs people are using, when they try to open an SVG that was created in a different program.
Unless you want to teach yourself what each and every program imports and exports at by default, and hope that whoever created the file didn’t change the default, so that you can scale the sizes when they get to your software, you’ll want to just use a 20" x 12" artboard and the GFUI should handle it for you, and it will stop rescaling the results in the GFUI.
Not going to work though without that size on the artboard. From whatever program you are using. (There’s a heck of a lot of historical discussion on this elsewhere in the forum, but finding it would be beyond me right now.) 
We do have the instructions for how to correctly save your files in SVG format here, to keep resizing from happening: