It's usually in the auto-import box. Some sites have the option for it I believe, others don't. If Prima allows it they'll be a little dropdown box where you can change it from full ring to 6 max.

I just checked in PT, underneath where you have the option to autorate players there a box you can check that says "treat all hands as" (or something to that effect. Check that box and there's a dropdown box where you can select 6 max to the right of that.