someone could help me to translate

"How will you have your coffee, long and black as usual?"
"I think I might have something different, flat white or some other milky one."
"An espresso, short and black with sugar."
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what is the meaning of "long ""short" in here? are they mean the size of the coffee cup? and what is a flat white? How do you say concentrated (less water)and plain (more water)coffee?
thanks for your explain