Think of a sandwich, the middle line tends to be shorter than the top and bottom. But please feel free to create your style.

I think they are about the same. Since it is the only character made up of three horizontal strokes, there is really no possibility of mixing it up with any other character whichever one is drawn longer. It really doesn't matter. So I believe there is no rule about it and anyone can write it as he wishes within reason. There are some characters for which the stroke lengths are important. For example 未 ("mi", yet) and 末 ("matsu/batsu", end), which are different only in the lengths of the two horizontal strokes. So the relative stroke lengths are crucial for them.