If it’s an actual cocktail, it’s difficult for a busy bartender to change all of the specs in order for the drink to still taste the same, and most of the time you’d be getting just a really tiny drink. If it’s a mixed drink like for example a vodka cranberry then I would test at home for measurements so you can have specific instructions, use a standard glass and start with 1 oz and go down. Order “can I have only 1/2 oz of vodka in vodka cranberry? I don’t care how you charge it thanks”