[Zeke]

A window should move in the direction of it's longer dimension. In the case of most double hungs, the sashes are equal and nearly square. That same window would be awful as a horizontal slider as the movable sash would rock back and forth.

A long window of short height should be slider or it could be an out swing awning. In a sense, awning windows allow you to clean the out side while standing inside. And you would have both sides of the window operable rather than a fixed side/ slide side.

By manufacturer direct. I they don't do that, ask for the closest wholesale distributor. Have them measure it, build it and if it won't install they have to tell you why you are not doing what you need to do or they did something wrong.

You can't lose on a deal like that.