Scaling (set of stairs) between two edges Hello Blender community :)

I have a slight problem I can't really figure out; 
How can I scale my stairs equally in the y-direction (green arrow), so that the beginning of the first stair will be snapped to the left edge of the face (since it is in wireframe mode, I'm talking about the dark green lines), and the last stair's end will be snapped to the right edge of the face. On top of that, the stairs should be equally scaled (first one should have same dimensions as all the other ones)

Note: a single stair's dimensions are currently (x=2, y=0.4, z=0.15)

Thanks in advance!
If I got what you asked, try this:

get the desired length selecting the mesh delimiting the available space (in the image it seems a sort of cubic shape), and copy the Y dimension in the properties panel.



Then select your stair made of array-copied steps, and in the same panel it should have a different value:



now paste the copied value in the Y dimension.



it should fit perfectly