Most often you will get small nymphs when you but a mantis. A 32 ounce deli-cup with a vented lid is sufficient and recommended to start as it is not only easy to control the temperature and humidity but it is inexpensive. It is what experienced breeders use for mantis nymphs and most average size species.
The container for holding the praying mantis must not be too big for it to find its prey and at the same time, it must not be too small for it will become too cramped. The length of the container should be 3-4 times the length of the praying mantis and the height should be at least 2 times its length. The reason for this is so that the praying mantis can have enough room to stalk its prey yet not too big or else the praying mantis won't be able to find its prey.