Yes, your scale seems to work although I'm finding the placement of trees are not as good as they were previously. Some are overlapping in, not ncessarily wrong positions but places that don't look too good. I've definitely seen at least one tree that was higher than some others yet overlapping them too.
The trouble with the slider is it makes the refresh slow and also all the trees are the same size still. I'd say that if you want to keep the slider then only make it refresh the image when "Generate" is pressed again.
That still doesn't fix the fact that all the trees are the same size though. I guess a quick workaround for that though is to change the brushes so that some of them are different size to begin with.

I thought your "Fill" button was broken to start off with but then I realised you were just showing the mask layer which is what I requested so good stuff on that.

I have some thoughts on how to solve your placement problems but I'll just go whip up a few example images first and you can then tell me if it's crap or it can help.