Welcome to the forum Pete. Your choice of terrarium (size and shape), as well as orientation, are greatly affected by which species you intend to keep. I like to keep imitator in pairs in vertical 10 g tanks (or the smallest tall Exo-terra or Zoo Med terrariums). For 2 pairs I would consider a vertical 20 g high or the 18 x 18 x 18 inch (45x45x45 cm) Exo-terra/Zoo Med terrariums. If you've more specific questions, please feel free to start new threads in the terrarium section.