When you order "tall" that means you want to weaken the drink by adding
additional non-alcoholic mixer.
If say you normally order a bourbon and coke (Poured, Highball Glass, Cubed
Ice) and decided to order it "tall" then you would get (Poured, Collins Glass,
Cubed Ice). The amount of alcohol would be the same (usually 1-1.5 oz) but you
would get more coke in a taller glass.