the peacock mantis would be a good one, females grow to about 12 cm males about 9cm. i got loads for sale at the moment. heres an ebay link but if you want to buy direct i accept cash, postal orders and cheques. £3 each plus £3 for postage for however many you want.
Orchid mantids are the prettiest but even in a small exo-terra you'd lose it and it'd have trouble finding food.
The best mantids for an exo-terra are the bigger ones, not as pretty but Hierodula and Sphodromantis are large, interesting and will give hours of pleasure watching them hunt and eat their prey.