Neogonodactylus oerstedii (what I have), Neogonodactylus wennerae both stay pretty small and will do good in 5g or larger. Something like a Peacock will need more room. You also need to keep in mind what you want your tank to be . If you are wanting a mantis in a reef type setup (no fish obviously) then your options for a shrimp are reduced by quite a bit. Many of them can not tolerate intense lighting , they will develop shell rot and eventually die.