The large size just doubles everything in both directions (a pixel would become 4 pixels). If you made it anywhere in between normal and large then you would need to use half pixels which don't work although theoretically it would be possible to rasterize a bunch of pixels that don't align with the screen pixels, it would be a lot of effort.
If the c++ visual controls don't have smooth scaling, the whole idea would have to be coded into the powder toy which is
jalfor:
...a lot of effort.
You pretty much answered my question. Oh well.
Edit:
What do you mean its possible, boxmein?
He means you can do it, but its hard to do and can be a hassle.