Are you planning on converting it to a vertical? And are you getting a sexed pair? Thumbnails can be tricky. Social dynamics can develop into problems within a group so yes bigger is better. I’d go with an exo terra 18 by 18 minimum. Aquariums are harder to orient vertically and the exo terra’s front doors make them more user friendly. It’s worth the extra money if you can swing it. If this is your first attempt at keeping darts though you may want to try a member of the Dendrobates genus or Phylobates first. They are much more forgiving.