In Ireland, the standard size of a flagon is 2 liters, but in most other places a flagon is not of any particular size. "Flagon" simply refers to a large leather, metal, glass or ceramic bottle or container used for holding liquids, especially wines and liquors.
A flagon is used in Anglican churches to hold wine to be consecrated. A flagon usually has a spout, handle and narrow neck. Roman Catholics use a glass or metal flagon to hold their wine. Post-2002, the pouring of wine from the flagon to the chalice was eliminated. Now the cruet, a smaller container, is used for the priest's chalice.