For me, I think somewhere there's a nice trigonometric curve that describes what I want where X = size and Y = complexity. Maybe not a directly inverse relationship, but close.
My ideal model railroad would have long stretches of single track on relatively flat terrain and many passing sidings. I would have enough industry to keep a few switching jobs open for ops, but mostly ops would be getting trains past each other on a modern(ish) mainline.
This is, of course, a nightmare scenario for a model railroad.
So for me to say I wouldn't want it to be that big....well that would not be accurate. However, I might have 75 sq ft of layout with only two turnouts. Will I ever have room for such a best? Not likely.
What is comfortable? I think a typical 10x12 bedroom-style layout with some strategic "near room" staging would be a good compromise, and might meet most of my goals.