The problem with SVG is that most programs assume that if you’re saving to SVG then you’re planning to use it on the web, so they use pixel-based dimensions rather than real units. (SVG supports real units, but for some reason most software that creates SVG files flat out refuses to use them when saving to SVG.)
Different programs have different ideas about how many pixels there are in an inch, so things change sizes.
- Tell Affinity Designer that you want your file to be saved at 96 dpi, which is what Glowforge expects.
- Set the size of your document to be exactly 20"×12". When the Glowforge software sees an SVG with that aspect ratio it ignores its usual assumption of 96 dpi and instead sizes it to 20"×12".
- Use PDF instead. It always uses real units so there should be no size confusion.
I use a preset in Affinity Designer that sets the document size to 20"×12" and the DPI to 96, but then I still usually save in PDF just to be safe. (Saving as PDF also gets around a few other potential problems, although it also introduces a few. But for most files I find PDF to be more reliable.)