Here it is, from this paper:
Proposition 5.1.1. The number of skyline polyominoes of area $A$ and width $w$ is $\left(\!\binom{w}{A-W}\!\right) = \binom{A-1}{w-1}$.
I'm referring to the first expression, not the binomial coefficient. I think it's a Stirling number of some kind, but I'm not familiar enough with them to be sure.