In this tutorial in the first couple of minutes it is explained how to set the dimensions of a cube and match the background images to this mesh. I have tried to get it right but till now I did not succeed. This is what the creator of the tutorial says in the comments:
I'm keeping only one mesh and just changing the x and y values of my background images you can see me add at 3:00. We want all the background images to match that one mesh.
Here are my images:
I think front and back images match to the mesh. But for the right, left and top view I only get one dimension matching. I don't see how you can match both dimensions matching without changing the mesh