Hello Nhu --There is a problem here related to the standardization of grit size of stone in soils. There are many such sets of standards for different sizes of grit, depending on usage. e.g., for lapidary work --grits are noted in terms of mesh size per inch for abrasives up to 50,000 + per square inch. For woodwork, etc. , there is more than one kind of grit size standard for sandpaper, wet and dry,and for stone sizes in rock and quarry materials for landscaping--and most of these scales are available and downloadable from the internet. Albert's idea of a photo would definitely solve some of these problems if there was always a scale in the photo however crude, such as a quarter or shilling, a cigarette, a finger or a ruler marked off in centimeters or inches. Using the plant as a scaling device doesn't work if the plant isn't known to most people, or is outside the usual range of sizes Putting a sample of the grit(s) in a teaspoon might be widely enough known to work quite well. What's universally recognizable and available to be workable?