Thank you all so much for your answers - wow, Aus Post has certainly changed things!

And 11dustyattic I do agree that the satchel size has shrunk - yesterday I blithely thought I was going to fit a neatly-tied bundle of four pieces of young child's clothing into a medium satchel (I remember fitting at least 6 in one of these a couple of years ago), but had to untie it and absolutey cram them in, then perform all sorts of contortions to get it actually sealed 😞  Thank you so much for the extra suggestions/info too - that's really interesting. I'll certainly try to have a satchel of each size to hand ready for next time so I can judge it better.

Cheers!