Think a size 6 or 8 inches round should do, actually depents on how big you want it, Definitely 3 layers, 2 layers will not be tall enough. I really can’t see any tapering in that cake picture but if you want to taper a 1 inch difference between the cake top and base should do. With the tapering a fabric ribbon won’t stay (lie down properly) at the base, so use a fondant ribbon. For the brim use gumpase and prop it up with either bits of foam cut to shape or crumpled up cling film, till it dries and takes on the curved shape – this should take a day (depending on your climate, takes longer in humid climates). Do remember to remove the props before handing it to the customer.