1. What does it mean?
In a coffee context, 아아 abbreviates 아이스 아메리카노. It refers to the drink, not a separate recipe or a special size.
Also written or said as 아이스 아메리카노, iced americano
2. The nuance
Because 아아 can also be an ordinary vocal sound, the cafe or ordering context does important work. The full drink name is clearer if the listener seems unsure. This shorthand specifies iced coffee but does not specify cup size or number of espresso shots.
3. Examples
“나는 아아 마실게. 너는 따뜻한 거 마실래?”
I'll have an iced Americano. Would you like something hot?
“회의 전에 아아 두 잔을 포장해 왔어.”
I picked up two iced Americanos to go before the meeting.
Sources
KoreanClass101: Korean language lessons
References support the meaning and usage. Example sentences are written for NoCapWiki.