The wheel and tire size debate seems to be a common theme in most disciplines of cycling. While some will say that gravel is best-ridden on voluptuous 650b x 50mm+ tires, it can’t be ignored that Unbound Gravel saw a lot of competitors opting for fast-rolling 700c x 35mm. In reality, there is no one wheel size or tire width to suit everything. The best gravel bike tire for you should reflect your terrain, riding style, and bike tire limitations. Luckily most of the best gravel bikes allow the option to run either 700c or 650b, so it is possible to experiment with different setups to see which works best for you.
Larger 700c wheels will generally roll faster on smoother surfaces as well as improve rollover capability thanks to their greater diameter. Smaller 650b wheels have become popular as they can allow a higher volume tire to be fitted beyond a frame's 700c max-width. Increased tire volume, when running lower pressures, allows the tire to absorb bumps better as well as yield more cornering grip thanks to the increased tire footprint. This makes them better suited to very rough terrain and bikes laden with bikepacking bags.
That being said, we are seeing many brands developing frames able to accommodate large volume 700c x 45mm+ tires and phasing out the need for 650b to run big tires.