An 8x8 will definitely be sufficient. I wouldn't suggest anything smaller in my opinion.
I tried looking it up online, but couldn't really get an idea of the polyester resin walls. Is building the mews out of wood an option? If not, as i mentioned, Home Depot and Lowe's sell plastic resin sheds. Is that something you would be interested in?
Nonetheless, you can always go the classical route and build it out of wood.