I've never seen a shot glass that didn't measure one liquid ounce, which is three tablespoons, which is nine teaspoons. Perhaps zenwriter drinks at different watering holes?
However, wikipedia says that in the US there are three common sizes, the Pony shot -- 1.0 fluid ounces (30 mL), the Single shot -- 1.5 fluid ounces (44 mL), and the Double shot -- 3.0 fluid ounces (89 mL).
I would guess that which size shot glass you select would depend on whether you're mixing a drink where the ratio of alcohol to mixer matters, or whether you're just monitoring how much you consume.
BTW, liqueur is a type of liquor. Check wikipedia for what makes an alcoholic beverage a liqueur.
Maryn, who doesn't have a single liqueur in the house