Flat vents are typically sized as which of the following?

Flat vents are typically sized as individual or dry vents. This sizing is crucial because individual vents are designed to handle the waste and air from specific plumbing fixtures without the need for larger stacks or systems. Each vent must be appropriately sized to ensure that it can effectively carry away gases and maintain the necessary air pressure in the system to prevent siphoning of traps.

Using individual or dry vents helps promote proper drainage and venting without the complications that larger diameter systems might introduce. Large diameter systems are generally used for more extensive plumbing networks and may not be practical for the specific function that flat vents serve. Vertical stacks, while essential in plumbing, serve a different purpose, primarily for carrying waste and venting from multiple fixtures, which isn't the focus of flat vents. Single units do not accurately describe the sizing that accommodates the collective needs of various fixtures, thus making individual or dry vents the correct answer.