Not gonna lie, I'm not really sure how to go about doing that. I read over the wiki and while some of the info there makes sense, I wasn't really able to figure out how to actually put these things together. I did use some of the mod's json files as an example template, though, hoping that would work. The one I ended up trying (and tweaking numerous times) was:
survival-blocktypes-stone-sand.json
[
{
"op": "replace",
"path": "/maxStackSize",
"value": 300,
"file": "game:blocktypes/stone/sand.json"
}
]
Out of habit every time I made a change I would update the .zip file, close the game and relaunch it, and go back into my test world. Never could get the sand to stack beyond 64. I've tried everything I can think of, though that was mainly just changing the "file" section, since the sand.json is in VS > assets > blocktypes > stone > sand.json.
Alternately I did check the games sand and sandlayered files, though I didn't see anything about maxstacksize in either one. I'm not above changing the base game's files if it's easy to do so, but given that I don't usually play solo, it's easier if it's in a mod that I can send to my wife so we both have the exact same settings, especially after the last game we played for a bit where literally everything was case sensitive and it wreaked havoc on us for a while.