Tiny mer that, giant mer this. Where’s my size shifter mers?
The mermaid who gets caught in a net and after the captain starts being all proud and everything and the mermaid just shrinks and slips through the net’s holes, much to the captain’s disappointment.
A merman who grows big enough for all the other mers to cling on as the group travels. Perhaps he also grows to distract predators or fight off threats to his non-shifter mer friends.
What about the classic trope of someone finding a merperson on the shore and taking them back to their beach house / lighthouse, putting them in their Tub. Maybe the merperson is unconscious for a bit but the lighthouse keeper comes to check on them and notices that they’re gone. Panicking they check around but the floor isn’t wet, then they see a small movement from the bath. Hiding behind a rubber duck is the merperson all tiny, not knowing what else to do but hide. Perhaps the keeper calms them down and calms them down, allowing the mer to grow to the size the keeper originally saw. Perhaps the keeper helps them heal and let’s them go after making a beautiful friendship only to one day meet the merperson again, but this time the roles are reversed. The keeper is saved by the merperson, and having been put on an unknown island. The keeper has no clue how they are gonna make it back but thinks the merperson can help them, the mer nods and grows bigger to make the journey faster, much to the keepers surprise.
Long story short, giant mers and tiny meds are great, but give me the shifters!