I think that 40-50 range is a good range where size starts to become a notable factor among eukaryotes. I think size would start having an effect before that point, but not enough to significantly dissuade the player from adding new parts. We can nail this down with balancing thankfully.
Changing the severity of the osmoregulation penalty across difficulties sounds good, but we should be wary of extending this effect to the AI too much - we don’t want harder difficulties to have generally smaller AI threats. Perhaps we can look into bolstering the surface area and volume ratios concept for harder difficulties to make strategy that much more important there.
As to the terminology, I wonder if “size-related costs” would be a fine enough blanket term. Surface area will be its own thing if we implement the SA:V ratio concepts.
Good point on the doubling of size issue. Is it possible to “freeze” the “size-related costs” stat upon exiting the editor, or would that muck things up? By that, I mean having it so that players exit the editor with their size-related costs stuck and unaffected by whatever happens on screen. So they’d keep their original costs and not have their cells dramatically suffocate/starve for no good reason once replication occurs.