See, I told you would get great adivice!
If I were you I wouldn't worry about the size of the terrarium unless you feel that there is a problem and as you say the gecko is moving about etc then just keep your eye on it. I would do as said in the previous posts and remove the substrate for now. Put the moss in a plastic container so it can be added to the terrarium but easily removed to be cleaned if you need it to keep the humidity up.
But the best thing you can do for it after that is leave it be for a couple of weeks! let it get used to its new home. you don't want to add to the stress by constant handling and opening and shutting the door lol
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