Scalable HTML with fixed maximum size. Possible?

For sure not out of the package, and I even doubt it would be possible with custom JS. You can publish to a scalable iFrame in JS, but I suspect it is not possible to give limits. However, maybe the JS gurus have an idea.

No.  The Scalable HTML option means that the course content will expand to take up whatever room is available on the browser window.  It will retain its aspect ratio (width:height ratio) but will take whatever room it can.  If you want to have a course at a fixed pixel width or height then you do not use scalable HTML.