Dab rigs are usually smaller than bongs because the potency of cannabis concentrates is so much higher that you don't need a large pipe. A dab rig should be less than 12" tall which is ideal for smoking concentrates. Most dab rigs larger than 8" are bigger because they include percs or recyclers. Dab rigs less than 8" tall are easy to use and perform well.
Gear makes a really good dab rig. So does Hydros. Nice Glass has recently introduced some awesome dab rigs as well. We have a large selection of dab rigs for sale online and at our retail location.
Some people buy larger dab rigs that have recyclers built into them allowing the smoke and water to cycle with constant filtration. This allows larger amounts of smoke to pass through on each draw. A recycler rig will usually cost more than a regular rig.