It should be no surprise that given the various uses of this term going back to 13th century England, there is confusion over its meaning today Historically it seems to have always implied large size, and most commonly, tankards had handles and lids The term also strongly suggested that the object was made of metal In 1984, SCI proposed that its members agree that the term would be used for "large drinking vessels, for beer or similar liquids, equipped with handle and lid, having a capacity of 2 liters or greater" This suggestion seems to have been met with resounding indifference Today the term is commonly used when referring to a large serving pitcher (with handle but no lid!), especially those made of glass (see eBay)
A tankard is a large metal cup with a handle, which you can drink beer from. A tankard of beer is an amount of it contained in a tankard. a tankard of ale