Bottled alcohol is sold in metric units, half liter, 750 ML, one liter and 1.75 liters as a convention, other measures are probably also available. For mixed drinks, the unit of measure is a jigger. This is not a standardized unit. It ranges from 1.0 oz through 1.5 oz, and tops out at 2.0 ounces.
There is also a convention using "a drink" as a unit of measure. This usually says one unit is one beer, one glass of wine (volume not defined) or one mixed drink (alcohol content not usually defined). Its a quagmire, and usually you can find the measures in a footnote if it is stated at all.