I suppose you could draw a box around the area you want, and then choose a specific tileset, similar to how the Garmin Custom Maps export works. It's likely to create some huge image sizes, but now that the tiling is there for Custom Maps, maybe that isn't much an issue.
I very much like how the custom maps export works by allowing choice of the output tileset. I don't find the original image export to be quite as intuitive, especially since a high resolution export involves a matter a faith - i.e., a large export is sometimes larger than what is displayed on the screen.
Correct me if I'm wrong, but as a larger coverage area is displayed on the screen the tileset may also change, contrary the user's intention.
The interface I would like to see would be more visual and would involve several components.
1. A choice of output size (as X by Y pix or aspect ratio)
or
2. A choice of drawing a box around a displayed area,
and
3. A choice of output tileset,
If #1 were chosen then #2 would be automatically created/displayed and vice versa.
KenF