Originally Posted by Jellby
The viewBox sets the coordinates of the svg canvas, they are not pixels or mm or anything, just numbers. It's telling that one corner is at coordinates (0,0) and the opposite at (293,333), so that when you make your image's size 293×333 it fills the whole space reserved for the SVG picture. You'll see why these numbers don't need to match the real pixel dimensions of the image.
Originally, Calibre had is using the numbers to fill the screen and if they were not the numbers for the image, the image would come out with the incorrect aspect ratio even if the code said to make the aspect correct. I pointed this out to Kovid and he fixed it so when Calibre generates the SVG code for the cover, it uses the numbers for the resolution of the cover. This could be a bug in ADE. But since most ePub is read using ADE, using different numbers (other then the resolution) may not work well.