Like I mentioned, the standard way to do this both in Unity and elsewhere is by using 9-slices: https://docs.unity3d.com/Manual/9SliceSprites.html. Using these would allow the window to be any size using the same set of images.
A potential update to this asset pack would add pre-assembled windows in the sense that the 9-slices are ready to drag and drop into Unity. After doing so, you would be able to follow the manual and resize the window as you please.
But it sounds like you want images of pre-assembled windows of specific dimensions? If that's correct, which sizes would you find useful?