I think they look great but I would have a problem with the variable length. If my DM gives me magic item cards, I will for sure buy a binder with card sleeve pages to put those cards in (no matter if they are cards or just paper). Having the variable length would keep me from doing that. I would prefer the text be variable font size with the card sized fixed to the standard 3 1/2" x 2 1/2".
Edit: I thought more about this and, if the main feature of your design is the variable length, maybe make that more obvious in the title/description. While it might not be to my liking, others may love it. Also consider adding some example 11" x 8 1/2" pages showing the layout you're going for using these cards. Marketing is (almost) everything.