Hi. I have a bearded dragon named Leonardo and he is about 8 months old and 14 inches long. Liz is eating all of his crickets and salad and he is pooping regularly. I'm super glad to join this community.
Welcome to the forum! I'd love to see photos of your buddy Leonardo. You can upload photos here https://www.beardeddragon.org/useruploads/ and post them to your thread using the XIMG button when you reply.
Their adult size can vary quite a bit. Usually somewhere between 18-22'' nose to tail end and anywhere from around 500-700g or so. A 40g breeder will hold an adult but a larger tank is almost always a good idea if you can. A good alternative if a larger tank isn't possible (or even if it is) is to provide areas outside the tank to roam around with portable basking lights set up.
A dome fixture either clamped to some furniture or hanging from an adjustable stand will work. You can use a regular basking light to provide heat or you could use a mercury vapor bulb to provide UVB too. The bulbs and fixtures would cost probably around $50-75 for a mercury vapor setup or a bit less with just a regular halogen heat bulb. It's a good setup to use for travel. Here is one of Darwin's setups (100w MegaRay MVB with an adjustable stand and dome fixture)
When it's time for a new tank it's most cost effective to build one on your own rather than buying one. This is a popular build that won't break the bank. https://beardies.dreamwidth.org/2145.html