Then since this is the first time you’ve uploaded one from this, it’s a good idea to check the scale in the GF interface once you upload it.
Measure the height and width of some portion (or all) of your design in your design app, then upload it. Once it’s in the GF interface, select the same portion (or all of it) and check the size with the ruler icons on the lower left. Sometimes you’ll find the scale to be off if the program in question is exporting it strangely.
I would probably urge them to consider exporting the design file with a 20x12” working space (which they should be able to specify when they generate the SVG). When the Glowforge web app detects that aspect ratio for the working space, it knows how to scale the file regardless of units used.