Shale, by definition, is a fine-grained sedimentary rock. This means the individual grains that make up the rock are too small to be seen with the naked eye.

Here's a breakdown:

* Clay-sized: The majority of the grains in shale are clay-sized, which are less than 0.0039 mm (0.00015 inches) in diameter.

* Silt-sized: Shale can also contain silt-sized grains, which range from 0.0039 to 0.0625 mm (0.00015 to 0.0025 inches) in diameter.

Therefore, shale doesn't have a single specific grain size but rather falls within a range of very small sizes.

It's important to note that the specific grain size distribution within a particular shale can vary depending on the formation process and the specific minerals present.