I just realized I might have made a silly mistake…
I created the mp.array but when changing the dimensions, I just assigned a new (width, height) tuple.
I think I have to assign it like var[0] = w; var[1] = h in stead!
I will try this out on Monday! 
What I’m still wondering though, is whether mp.array works when it is a property of an object… We’ll see!