Which size to lead with
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750 mL is a convention, not a rule
It is one of twenty-five authorized sizes and it carries no special status in the regulation. It is the default because distribution, pricing and shelf space all organised around it decades ago.
Which means deviating is legal and expensive
A 700 mL bottle is authorized and is the European standard. Selling it in the US is lawful and puts you at a disadvantage on a shelf where every comparison is drawn against 750.
375 mL is the trial size
Half a standard bottle. It lowers the cost of trying an unknown brand, it fits a gift set, and it is the size a bar will accept for a tasting that would not take a full bottle.
50 mL is the sample
The miniature. It is a sampling tool, a gift-set component and in some markets a product in its own right. The catalog holds 8 containers at 50 mL.
475 mL and 945 mL are the interesting ones
Both are authorized standards of fill and both are well stocked here: 110 containers at 475 mL and 59 at 945 mL. Neither is a size the category defaults to, which makes them worth knowing about rather than ignoring.
Where they come from
They are the metric equivalents of a pint and a quart, carried forward into the authorized list. For a product that wants to read as traditional or American rather than as a standard liquor bottle, that lineage is a real asset.
Lead with one, not three
Every size is a separate bottle, a separate label layout, a separate case count and a separate line on a distributor sheet. A first release is one size, and the second size is a decision made after the first one sells.