The size of an 8-ounce glass is dependent on the style of the glass, but each glass has the volume capable of holding 8 fluid ounces, 1 cup or 236.5 mL. If stated to be an 8-ounce glass then it is a relatively small drinking cup.
The size of a glass may be dependent upon the design and the material it is made out of. Most glasses made out of glass tend to be larger and heavier, relative to their volume capacity, while glasses made out of synthetic materials tend to be lighter weight. 8-oz glasses are normally used for serving cocktails.