have a look here: http://www.panamas.co.uk/info_hatsize.htm
according to that, you need a 7. although i think you should go and try them cos different makes can be slightly different shapes, and it has to fit just right, imho.
I honestly wouldnt ever buy a hat online... you dont know whats happened to them! It could have been dropped several times from thousands of feet and you would never know... until you have a damaged head from buying a damaged hat...
I have a charles owen XP keep it for posh and it's lovely although prob best to go to a saddler and try some on as I found that Charles Owen suit me but Champion don't think they all fit slightly differently, what suits one person might not suit another!