Different sized gable, within gable.

[Elovia]

... I understand that to change the heights of the gables you have to change the ceiling heights of the rooms?

Yes and no. For a roof plane of a given pitch, as you increase the base width, the gable end ridge height increases. It is a matter of geometry.

As for your unasked question of how to do it in HD 8 ... I don't know. I'm not conversant enough in the limitations of HD to venture a guess on what you can and cannot do. I do, however, think that the roof will be a bit of tricky business to get it just as shown in the picture given the limited tools you have available. Perhaps someone else here who is more conversant in HD can guide you.

Were I to approach the problem, I would probably start by defining the end walls similar to how you've drawn them in the red overlay, and then adding the entry and porch later. The guiding rule of thumb I use for my initial approach to most designs is to establish the exterior shape and roof lines first, and then finish the interior.

I'm sorry I couldn't be more help.