Eeeeehhh that's a tough one. Size can come down to more than just age factor. Why don't you try calling and finding out - I'm presuming (though could be wrong) you will have signed and given information to buy a pet (a legality here anyway) and so they can look back on the records to see what little guy you've bought - even if it was only yesterday if you're lucky enough you can speak to the same person who will remember you.
Failing that, I'm pretty confident that 6" give or take a few cm would be around 6-7 weeks old.