I totally agree with the default sizes as far too often people don't read properly, and are hard to get hold of.
I have a similar issue with my "choose any 3 coloring pages" listings. I simply add a line to my description stating that if no choice is entered then pages 1-3 will be sent. This places all the responsibility with the customer and I've not had any complaints yet!
For now, try a few ,ore times, both by email and by etsy convo. Remember not everyone is as glued to their email as we etsy sellers :-)
If the items are not one of a kind I would not rush to cancel the sale, as you can relist the item and won't be losing potential sales.
Good luck!