Depends on the engine. Going by the MAF adapters from Draxas, the Vulcan uses a 3" filter opening, and the Duratec uses a 3.5" filter opening. The physical size would depend on where you want to mount it, clearance in that area, etc. Almost any filter will flow enough air.
Even the smallest K&N cone will outflow the stock paper filter many times over. Don't worry about going too small. As Bob also mentioned, if you are using the DuratecPerformance/DRAXAS MAF adapter (or almost any other MAF adapter) for the Vulcan, any filter with a 3" Flange will bolt right on and *presto* you have an instant HAI (aka Short Ram).