General consensus is about #9 - balance between casting heavy/big flies and getting fish in quickly without undue stress. Saying that, my limited attempts haven't seen my pike fly gear tested as yet!

Will give it another go down a local river this year though (have had plenty spinning over the years, just not on fly gear)
Was reading an article in Anglers Mail the other day, some chap had been catching plenty, big doubles too, from Wyrley & Essington cut on fly gear. Have also spoken to other pike fly anglers who reckon its better than spinning gear where its do-able, because the flies are soft and more mobile and pike don't tend to spit them out like they sometimes do with hard lures - makes sense to me.