Pipe sizes used in farm irrigation systems often vary depending on region, crop type, water source, system design and farm size. Here are some common farm irrigation pipe sizes, usually measured in inches:

1. 4-inch (101.6 mm): 4-inch pipe is a common farm irrigation pipe size suitable for small to medium-sized irrigation systems such as vegetables, fruits, and short-term crops.

2. 6-inch (152.4 mm): 6-inch pipes are used for medium-sized farm irrigation systems and can be used to grow some large crops such as corn, rice, and wheat.

3. 8-inch (203.2 mm): 8-inch pipe is suitable for large-scale farmland irrigation systems, usually used for growing large-scale crops such as cotton, grass, and pasture.

4. 10 inches (254 mm) and larger: These large-sized pipes are typically used for irrigation on very large scale, such as large farms, orchards, and plantations.

It is important to note that the selection of pipe size also depends on the design parameters of the irrigation system, including the required flow rate, pressure, pipe material and pipe connection method. Additionally, consider the pressure capabilities of your pipes to ensure they can withstand the water pressure in your system.

When designing and selecting irrigation pipes, it is often necessary to work with a professional agricultural irrigation engineer to ensure that the system can meet specific needs, provide sufficient water volume, and perform well in terms of efficiency and reliability. This ensures that the farmland irrigation system operates smoothly to meet the growth needs of plants.