Diameter or length? Are you batch sparging or fly sparging?
For diameter: I would not go smaller than 3/8" and would prefer 1/2" or 5/8". You can always cut down the flow rate with the valve, but you can't increase the flow past the capacity of the nipple/valve.
For length: I'm guessing a 2" nipple might be a little too short to catch threads if you have any external washers. Going with a 2 1/2" may be a little too long, but washers can take up that extra length pretty quickly if you don't mind the look.
perhaps a line of single hop IPA's - there's so many new hops out there!!!