Rigid Pipe Diameter
The standard rigid pipe used to carry water away from gutter downspouts is nonperforated Schedule 40 PVC pipe in a 4-inch diameter. Before purchasing the pipe check with your local building department to find out if there are any rules requiring a different type or size of pipe for this purpose.
Also, if your gutter downspout pipe is very large, choose rigid pipe that is as wide as, or wider than, the downspout. Choosing narrower drainage pipe could cause a clog.
Flexible Pipe Diameter
Flexible drainage pipe can be used to go around obstacles, but it cannot be cleaned out with a drain snake. For this reason, you should avoid using it whenever possible because the amount of debris that flows through gutters may clog corrugated pipes.
To discourage clogging, use flexible pipe that is wider than the gutter downspout. Flexible outdoor drainage pipe is available in diameters ranging from three inches to eight inches, although 4-inch diameter pipe is the most commonly used size.
Be sure to purchase nonperforated pipe.
Gutter downspout adapters are usually 3 inches by 4 inches on the rectangle side, although they can be purchased in sizes of 2 inches by 3 inches or 4 inches by 6 inches. On the circular side, they vary from three inches in diameter to eight inches in diameter.
These measurements are usually estimates only, and you should measure the pipes, gutters and the adapter you plan to use before purchasing.
Length of Pipe
The drainage pipe should carry water from the gutters to either a sewer system or far enough from the structure that there is no danger the foundation or basement will get flooded. The pipe should also slope down away from the house at a minimum pitch of 1/4 inch per foot of pipe.
Calculate the length of pipe needed to accommodate both of these requirements.