A bearded dragon is a desert lizard, and it can't warm itself from the inside. It warms up by walking to a hot, bright spot and cools down by walking away again. That only works if the enclosure (a vivarium, or "viv") is long enough to hold a real hot end and a real cool end, with space in between.
The RSPCA says the minimum for one adult is 120 cm long, 60 cm deep and 60 cm high. A vet hospital's handout (SASH) gives a handy rule for growing dragons: the floor should be at least four times the dragon's length, so a 60 cm dragon needs a floor of about 240 x 120 cm. Put in your own measurements below and see how your setup compares.
A small viv isn't just boring. Heat and light are packed into a tiny space, so there is no cool end to escape to. The dragon can't pick the temperature it needs, and that's the whole point of the setup. A longer viv also lets you run the UVB tube (the special light that lets a dragon make vitamin D) over about half the length, so there is a bright zone and a shady zone.
The RSPCA lists the numbers a good viv should reach: a hot basking end of 38-42 °C, a cool end of 22-26 °C, and nights that don't drop below 20-22 °C. The SASH handout gives slightly different numbers (a basking spot around 38 °C, and a night low of about 23-24 °C). When sources differ by a few degrees, it's a sign to measure and ask a reptile vet, not to guess.
Both the RSPCA and the SASH handout say it is best to house bearded dragons separately. They are territorial: males chase off other males, and females in groups can fight and injure each other. A bigger viv doesn't fix that, so plan the space for one dragon.
Keep the viv away from a window, because glass blocks UVB. Avoid loose "calci-sand" and loose bark or wood chips as flooring; the RSPCA warns they can be swallowed and cause blockages. UVB tubes fade over time, so follow the maker's replacement schedule.
Measure the viv, measure the dragon, and give it at least 120 x 60 x 60 cm, and more as it grows.
A reptile vet is the right person for health worries. Low UVB can lead to metabolic bone disease, where bones weaken; signs include muscle twitching, swollen legs and fragile bones. Also call a vet about weight loss, odd droppings, a dragon that stops eating, or a female who may be egg-bound (a serious emergency). It's also worth booking a check with a reptile vet soon after getting a new dragon.
Thinking of getting a dragon? Check reptile rescues and welfare groups first.