Benford dories often have inferior rudders. We had to completely redesign and rebuild Badger's and the one that he shows in the drawings of Badger, as distinct from the 34 ft dory, is not the one we fitted.
Dories are notorious for having low initial stability I should not want to put a lofty mst on one. Follow the rudder and rig on Jim Creighton's dory of similar size, and you won't go too far wrong, in my opinion.
As for dimensions: when we designed the schooner rig for Badger, we just followed the formulae. I don't think H/McL say you have to desing different sized masts depending on how many there are, but I could be wrong.
Thanks for your comments Annie. Yes, I've followed the discussions on rudders for Benford dories and have modelled mine following the recommendations as I understand them. To give Jay Benford his credit, the 31.8ft dory that I'm building was (I believe) the latest version he designed, and the rudder on the plans is very much in keeping with the rudder design thoughts I've read elsewhere. I have added end plates and an airfoil cross section, and hope it'll turn out to function - only time will tell!
You are quite right that H/McL don't say that mast size is different for more masts. They do, however, base their mast diameters on sail area against mast height, which is not what others here do. I note that Pete seems to base his mast diameters on H/McL derivatives and therefore don't doubt that their suggestions work.
I have to say that I'm very impressed with the tabernacle design you've used on FanShi. That, coupled with the hybrid mast construction concept, is what started this whole line of inquiry for me. I'm still exploring ideas... :-)