Hi Cpiasecki,
Thanks for your reply. Here is a screenshot of the model driven app, with an embedded canvas app on one of the tabs:

The canvas app starts halfway in a large grey portrait block.
Unfortunately I have no options for setting a custom image size:

Although, I'm able to deselect "Scale to fit". Now the app shows in landscape in the model driven app.
However, that doesn't meet te requirements, so I'm still looking for a landscape solution with Scale to fit set to Yes.
Thanks again,
Elowy.