When it comes to the festive season, it's go large or go home. With that in mind, we've thrown together our complete listing of magnums for you to crack open and share this summer. We all know there are few sights more welcome in a bathtub of ice than a towering 1.5-litre bottle of Riesling, and we have the best in the world from the likes of Dönnhoff, Heymann-Löwenstein, F.X. Pichler, Frankland Estate, Gunderloch and Georg Breuer. But then Champagne en magnum is a must,…