You must remain size medium and you must keep the same basic shape (your DM might interpret this but the example given is you can't change from bipedal to quadrupedal). You can only change into a creature you have seen.
Your clothing and equipment don't change so if you change to something like a firbolg you will either need some firbolg sized clothes or go naked.
Note that your ability stats do not change with the transformation
I’ve created a Changling but don’t think understand change appearance? What are the limits to this?
You must remain size medium and you must keep the same basic shape (your DM might interpret this but the example given is you can't change from bipedal to quadrupedal). You can only change into a creature you have seen.
Your clothing and equipment don't change so if you change to something like a firbolg you will either need some firbolg sized clothes or go naked.
Note that your ability stats do not change with the transformation
You can look like any medium humanoid, basically.