This is based on a situation that’s happening in my current campaign, though the example below is simplified to remove unnecessary context.
The shapechange spell (Player’s Handbook, page 274) only limits the form that its user can change into by Challenge Rating, rather than size.
In the following situation, how, if at all, would shapechange be suppressed?
A level 20 wizard and a level 20 druid stand with 10 feet of space between them (figure 1).
To begin their battle, the druid casts shapechange, transforming into an ancient brass dragon, a Gargantuan creature (figure 2).
To counteract this, the wizard casts antimagic field (Player’s Handbook, page 213) which creates a field of anti-magic in a 10-foot radius sphere around the wizard within
“Spells and other magical effects, except those created by an artifact or a deity, are suppressed in the sphere and can’t protrude into it.”
It seems clear-cut that shapechange is suppressed until the druid exits the antimagic field, and so reverts to its normal, medium form (figure 4).
Now the druid’s smaller, Medium form, is no longer within the antimagic sphere. So it should regain the effects of shapechange, transforming back into an ancient brass dragon—which immediately puts its form back into the antimagic field, repeating the process. If it were another buff spell, such as fire shield (Player’s Handbook, page 242), that doesn’t affect the target’s size, this would be a nonissue. But shapechange, along with enlarge/reduce (Player’s Handbook, page 237), polymorph (Player’s Handbook, page 266), and any other spell that might increase the target’s size run into this issue.