I know that the upcoming removal of Size-based visuals is super sad for some. Here's a few things that can be done to mitigate people's disappointment.
Go by common Marvel Height statistics
Take the primary forms of each character (i.e. the default costumes) and set their height accordingly. Shulkie for instance, should be a head taller than Cap. Wolverine should be about 5'3", Hulk should be about 7-feet. etc.
(re)Design characters that have variable height with this in mind
Hulk, Antman, She-Hulk, etc. should have their visual design made to take into account size increase/decrease and bind those aspects of their visuals to the "traits" syste; not just "temporary" grow/shrink, but actual static size differences based on power selection.
Dynamic Camera (option)
Some zones (like the rooftop) have a nice close-up camera that is appropriate since the surface area is small an manageable, where as patrol zones basically require you to have max-camera distance to not be punked by something off-screen. I think an option for a dynamic camera that zooms in/out depending on action could be a way to visibly enjoy your character when not in the midst of chaos. If you are attacked by something beyond your field of vision, the camera should (gracefully) zoom out to include that mob on your screen, then when out of combat or not attacked outside your field of view for a certain duration, the camera should (again, gracefully) zoom in on the action. The effect should be gradual and not disorienting, but it could go a long way to compensating the people who used size increase to visually enjoy their character's costumes they spent so much money on.
Costume height differences
Some costumes should be built around different physical sizes. Some versions of Hulk (savage) are larger than others... Jugg's has different sizes in different universes, Thing, different Spider-man's (men?), Wolverines, etc. Let the character "version" match the height from their respective universe.
Character "vignette" transparency (option)
This is an effect I know I've seen before. A small area around your character (i.e. either mimicking their silhouette or a circle/oval) should be view-able through the chaos. i.e. No matter how many mobs are right on top of your toon, you would be able to see yourself to a certain degree. Again, this should be an option and not mandatory.
These are just a few options on how to compensate for the loss of the Size-based visuals. Let me know your thoughts below.