I like a skinny ruler with both mm and 1/16th inches.
Is there some very widely known, uniform size seeds like petunia, broccoli and radish? Then we could say "a little bigger than petunia" or "almost as big as broccoli".
We could glue 10 seeds of each "standard variety" onto white index cards and share them around, like the National Bureau of Standards.
Or, here's another idea that is technically accurate, but useless to 99.9% of gardeners. I mean, who has 5-10 grades of wire cloth handy?
If you have a collection of hardware cloth in mesh sizes like 60, 30, 26, 22, 18, 16, 12 and 10, you can make a stack, drop the seeds in the top, and just shake. Then look to see which screes they passed through, and which screens are holding most of the seeds.
Then the size could be reported as "mostly passes through 22 mesh", or
"stops at 26 and 30 mesh".
(22 or 30 wires per inch, square openings around 0.045" or 0.033")
- Most window screening is 24 mesh (I think) I found some at 18x14 threads per inch
- Kitchen strainers are mostly either around 18 mesh or 30 mesh.
- Fabric stores have cheap "mesh" fabric at 8 threads per inch, and maybe some other sizes. - You can make a nice strainer with windows screening or fabric plus an embroidery hoop.
Maybe, for medium-small seeds, we could photograph them with a "standard window screen" of 24 mesh. Then for small seeds, you could see how many fit loosely into a .042" square (1.06 mm square). For larger seeds, you could compare them to what is almost a 1 mm grid.
