Bearded dragon cages are surprisingly diverse, and because of the fact that bearded
dragons are desert dwellers and so don't require high humidities, you have one of the widest ranges of cages available.
Species like green iguanas need a humid environment and so ideally need an enclosed cage in order to keep the humidity up - but
that's something you don't need to worry yourself about with bearded dragon cages.
For baby bearded dragons, a glass fish tank (without water of course!) of around 2-3 feet in length (60-90cm) is ideal, with a mesh
lid to fit over the top onto which you can fit your heater and
lighting setup, yet will allow fresh air to circulate and prevent your bearded
dragon from escaping is ideal.
Unfortunately, virtually no online suppliers will deliver fish tanks direct to you because
of the weight, and hence cost of doing it.
However, to be fair, fish tanks are easy to get hold of from just about any petshop, whilst the mesh screen are far harder to find.
Larger bearded dragons should of course be kept in larger aquariums of 120cm or so in length.
Your other option is to consider one of the special mesh cages which are ideally suited to bearded dragons, with the only downside that so much fresh air circulating through the mesh means a slightly higher bill
for you in terms of heating the bearded dragon cage.