If you're reading this while creating initial designs for your distribution system, then you're on your way to avoiding some costly compressed air piping sizing mistakes. If you're reading this while redesigning or re-fitting your compressed air piping, then you may already know how frustrating those mistakes are.
First, determine your compressor's maximum cubic feet per minute (CFM). It's easy to underestimate the size of piping you'll need if you're not careful. If you choose piping that is too small, you'll quickly see that it can lead to pressure drops in excess of 10%. This can greatly reduce operating efficiency, causing your compressor to consume more energy and costing you dollars. CFM is arguably the most important factor in determining piping size. For a hypothetical example, an air compressor with a CFM of 1 will always suffice with half-inch piping, even if the pipe has a length of 300 ft (not taking into account the added length of possible fittings).
Second, consider what fittings you are planning on using. Draft a schematic of all pipes, valves, fittings, and other important features so you have a solid idea of what features you want and how they best fit together. Remember that the length of each pipe the air will travel through correlates with the amount of increased frictional air loss. Any turns or branches in piping increases pressure drop as well. If you're still planning and not sure how many bends or fittings to account for in your pressure drop calculation, the rule of thumb is to multiply your base estimate by 1.7.
Third, anticipate any future needs you may have for adding on to or changing the distribution system. Larger piping now will mean better accommodation for adding on parts later. It may feel like an investment now, but paying a bit more for a single install of more materials is a lot less expensive than paying a second labor fee later. Not to mention less installations is less wasteful and more time-efficient.
The main takeaway: it's better to overestimate on pipe width than underestimate. Pressure drops add up to be a very costly problem in a large system with lots of fittings that have gone unaccounted for. Lastly, double-check and triple-check calculating your estimates. The adage "measure twice, cut once" can apply to compressed air piping sizing as much as it can to carpentry.