The person you're replying to posted a link which mentions in the "materials and methods" section that the particles in the study were all smaller than 2mm (and 99.6% smaller than 1mm).
To compare with your pebble, which wikipedia says is
> A pebble is a clast of rock with a particle size of 4-64 mm
If you open the article and click the only link in the first paragraph, leading to the source, it will tell you in the introduction
> Crushed metabasalt and olivine soil amendments were used to investigate changes in soil pore water alkalinity as a proxy for bicarbonate production. The metabasalt (median grain size: 102 ± 22 μm) was selected because of its benign elemental composition and modest weathering potential (the parent volcanics includes felsic facies, yielding an overall silica content in the intermediate-to-felsic range). The olivine (median grain size: 83 ± 12 μm), in contrast, was chosen for its potential to weather rapidly
TL;DR scan for the digits in that quote. This stuff is measured in micrometers.
> Why would you even study such things?
Because if everyone were willing to read sources and get to understandings, we would probably not have climate problems in the first place
