Reptiles are far less active than mammals or birds and require smaller cages when compared with a mammal of the same size. My King Snake and Rat Snake both spend a large proportion of their time in a hide box. My Boa Constrictor utilities less than a third of the total floor space of her cage. My Bearded Dragon is more active and spends more time roaming the cage than the snakes. My Leopard Gecko remains hidden most of the day and is only active at night. Choose the size of the cage in accordance to how active your reptile is. Larger cages for more active reptiles.
Reptiles are not social animals. Bearded Dragons can be kept together but it will not increase your Bearded Dragons quality of life in any way. Snakes should not be kept together as fights over food will occur and some species eat other snakes including members of their own species. Rather have several smaller cages each housing a single reptile rather than one large cage housing multiple reptiles. If you still decide to house reptiles together the cage must be bigger.
Bigger cages take longer to heat up and smaller cages heat up quicker. A 1.2m x 60cm x 60cm will need a 100-150 watt heat lamp to produce enough heat for a Bearded Dragon. A 90cm x 45cm x 45cm will become an oven if you have installed a 150 watt light bulb in it. Take this into account when purchasing a cage.
The space available in your home will affect the size cage you buy. Always make sure you have the space in your house for the cage your reptile needs. Rather get a Corn Snake instead of a Burmese Python if you live in a two bedroom flat.
Certain species prefer smaller cages. Ball Pythons, Blood Pythons and Sand Boas are more likely to eat and will actually thrive in small cages. Many snake species do very well in plastic tubs in a rack system. Baby snakes do not do well in huge cages.