What are the dimensions of a standard...

The reason you couldn’t find an answer from those folks is there is no standard rain gauge. The commercially produced rain gauges are simply collection devices with calibrated markings. Making one yourself is a great exercise in construction and applied mathematics. The rain gauge should have a collection area of at least ten times the area of the measuring device. First, use a ruler and pour water into a one gallon container, such as a used bleach bottle, to a depth of 1 cm. Pour this water into a tall jar and mark water level with grease pencil or paint. This will be the mark for 1 cm of rain. From this first mark you can measure and mark up tenths of centimeters all the way to the top of the jar. Now you’re ready to capture rain. Avoid having trees or buildings in the way, and don’t place the gauge where channeled winds, such as a passageway between buildings, would disrupt the rain. The gauge should be over level ground and at least twice the distance away from trees and buildings as they are tall. For example, the gauge should be at least 12 feet away from a 6-foot tree, and the top of the gauge should be at least three feet above the ground.