The numbers are prawns per pound. So the smaller the number the larger the prawns. U means under (less than). So U10 size prawns are <10 per pound, so likely 5-6" long prawns.
This shows U8 Tigers and 10/15 Endeavours next to a very large lamb chop.
http://i124.photobucket.com/albums/p.../Prawns001.jpg
http://i124.photobucket.com/albums/p.../Prawns001.jpg

