Checked with Australia Post, the minimum size for a postcard to be considered a postcard is 88x138mm, so my 89x140mm juuuust scrapes by.

I've mailed a couple 5x7s to the US as well and they apparently arrived, so as long as it complies in the country of origin it seems to be OK. I've sent a lot of nonsense through the mail myself, but I'm a little paranoid this will be the one time I make a silly mistake and 50-odd postcards get lost in the ether.