Hi Spangodrum! A Full Imperial sheet is, I think, around 22 x 30 inches, so cut into 4, each quarter would be about 15 x 11 inches. You'll obviously need to know the inside measurement of the SAA 20 x 16 double mounts, so thats the first thing to ascertain.
I think you'll find that the "5mm offset" on a double mount is the amount of "slip" showing on the mount. Sometimes this is a different but harmonious colour so that if the mount is, say, 20 x 16 outside size, there will be a 5mm "slip" showing around the inside of the mount, next to the painting. personally, I think double mounts, whether all the same colour, or with a different coloured slip, enhance a painting more than a single mount.
I hope this helps, but come back to me if you need any more clarification.
Very best wishes,
PA & RSC Wigan area