I recall a story in the news a while back, where some Continental-style café/bars in the UK were trying to sell beer by the 500 ml glass, as is traditional in Europe. The law came down on them like a ton (or tonne?) of bricks and said they were not allowed to do that.
I have to admit that I find that baffling. No, not baffling, pointless and stupid. It's a pointless and stupid law made by pointless and stupid legislators.
On a related note, can any US members remember buying a pint of whiskey?
Liquor comes in specific size containers, all metric, but many are meant to be close to imperial sizes that
alcoholics we have become accustomed to.
250 ml
375 ml (often called a pint)
500 ml
750 ml (quart)
1000 ml
1750 ml (half gallon, though the really old guys call it a handle.)
EDIT: in bars, poured liquor comes in units of a "shot" but nobody knows how much that is!!!

My point is that nobody tries to dictate how much gets poured at a time. You can ask for a mixed drink with 1 shot, or sqrt(2) ounces, or any reasonable number of milliliters. The bartender will probably complain that they can only measure shots, but no pointless stupid laws dictate that they can only dispense in shots
Beer (in bottles or cans) is still sold by the ounce. In some cases the traditional 12 oz size has been replaced with 11.5 oz. Microbreweries sell refillable 1/2 gallon (64 oz) bottles they call a growler.
Draft beer at bars are by the glass, sometimes with large and small options, sometimes with the large being a full pint. No regulations, except possibly for some maximum size that I'm not aware of.
It all goes down the same for me!