Size, handle and wall thickness
The range here runs from 28 to 60 cl, and in practice that means two categories. The 28 and 29 cl tankards are the small size: a half, handy when the beer needs to stay cold because you finish it quickly. The 50 to 60 cl models are the pint size and the classic tankard size, and that is where the emphasis lies.
Watch three things when choosing. The handle should leave room for four fingers, otherwise the weight rests on your knuckles. The base may be thick, because that keeps the beer cool longer and makes the glass tip-resistant on a crowded table. And the wall determines the weight: thick-walled pressed tankards already weigh a fair bit empty, while thinner-walled models drink more pleasantly but insulate less.