You are adding the first dimension that will change the X length of the rectangle. It is asking if you want to scale the entire sketch, which will change the Y length to keep the same proportions the rectangle has now; or just change the X length, leaving the current Y length as it is now. This is more important with complicated initial sketches where a drastic dimension change can flip some of the geometry in an undesired way.
I usually press "yes", especially if the current dimension value is significantly different from my desired dimension.