There are several species of large barbs available to the fishkeeper, whether you've been keeping fish for years or are a complete novice. Aquatics outlets are usually more than happy to sell any of these hefty barbs to hapless newbies without a second thought - and are of course happy to take back the fish once they have grown too large for your tank.
Most common of these are the silver shark (also know as the Bala shark, balantiocheilus melanopterus). These are streamlined silver fish with striking black bars on the edges of the fins. Deceptively small in stores, these fish can reach up to 16" - in an aquarium, 12" is more likely, but still very large for the average tropical community. Being very active fish, they need a very large tank for their size - 6' x 18" x 18" is an absolute minimum. Balas are notorius for jumping out of the tank, and the stresses of cramped conditions make this more likely.
The next most commonly sold large barb is the tinfoil (barbus schwanenfeldii, a tall silver fish with red fins). The profile is similar to the Bala shark - these fish reach 14" quite easily, and need a very large tank to accomodate their high activity. Again, 6' x 18" x 18" is a minimum, bigger is definitely better.
The third large barb currently rising in popularity is the lemon-finned barb (hypsibarbus wetmorei, or h. vernayi). Also called the lemon tinfoil, gold tinfoil or diamond shark, these fish are the biggest of the main three - in an aquarium sizes of 18" are typical, and they can reach 20". In appearance they are very similar to the tinfoil - tall and silver, with golden yellow fins instead of the tinfoil's red. The scales are larger with a dark outline. They are peaceful fish and not as active as the bala or tinfoil - however, the enormous adult size requires an equally enormous tank. 6' x 24' x 24' should be considered the bare minimum, and if a larger tank can be afforded then it is preferable. These fish grow very fast as well, so choosing the bigger tank from the start is advisable. Care should be taken to ensure adequate aeration - lemon-fins are very demanding of oxygen, so the more aeration you can get in the tank, the better.
There are other fair-sized barbs sometimes seen for sale - spanner (or T) barbs (puntius leteristriga) are often seen in stores. The maximum size for these is 7", so they are within reach of medium community setups and are quite striking to look at. Clown barbs (puntius everetti) are rare to find, but very pretty fish - reaching 5-6" (females slightly larger), they are a good size for community setups. They need peaceful tankmates, however, and must have other clowns to shoal with. This is true of all the barbs in this guide - all are shoaling fish, the clowns and lemon-fins in particular. The spanner shoals less as it ages, but still prefers company of its own kind.
Be aware when you see the largest 3 barbs for sale - they are beautiful fish, and well worth the effort if you can afford a big enough tank and heaters. They are, however, messy fish, so a good filtration system is essential.