The description of force cage includes a rule for what happens when creatures too large to fit in the area are caught in the spell's area (they simply get pushed aside). Given this I think it's unreasonable for a player to claim that the DM is gimping their character by ruling a creature is too big. The expectation that not all monsters are trappable is set up right in the spell description. It's totally up to the DM to determine which monsters fit into the area and which do not.
The more power in the DMs hands to make up things as they happen, the greater the percentage that a player will get screwed over somewhere. I've seen plenty of DMs screw over players in 5e already watching streams and such.