I just keep it simple and assume the Vial size from the Equipment List is the standard for one dose of a potion. The Vials holds 2oz. which I find reasonable since you should have no problem gulping down that in one round.
But you bring out a very interesting question on when you have a container holding several doses. We have normally not bothered and assumed that you instinctively drank the right amount. Maybe from feeling some effects from the potion when it starts to do it's thing.
Ofcourse as already mentioned, potions could be in almost any form.
Potions come in several forms, from watery liquids to oils, balms, and even incense.
But in whatever form I do still use the weight of 2oz dose as a guideline. Ofcourse a gas would probably take up a larger volume than 2oz of liquid and so on. But the weight should be the same. I know someone did suggest pills as an easy way to carry around ready to use doses. But with the weight from 2oz would probably result in a cookie size pill which I am ok with.
I forgot to mention that the vial itself weighs 0,25 lb (4 oz) so a filled potion bottle would equate to 6 oz. But I just make it simple and assume 0,5 lb for a full vial.