Is it really important to see the size of the image?

I think so because it is scaled to be visible in the photo panel regardless of its resolution.

Regarding the transparency, that's not a biggie either, but it could be an option?
But please don't abandon Sunflow quite yet.

I do not intend to abandon Sunflow but I prefer to leave the choice of the rendering engine to the end user as in this old plug-in by adding YafaRay. This will imply a change of the interface by favoring the display of advanced settings.