Salt size is more about dissolution rate. Always measure salt by weight if possible. A finer flake salt will, of course, dissolve faster. But sometimes you don't want that to be too fast, but gradual.
What happens when you surface salt a cheese or curd is that it draws moisture out from the outer layer of the curd and creates a sort of brine layer all around the cheese. So it brines, but not in a full soak brine, more like there's movement of the salt into the cheese and water is drawn out of the cheese. As that water is drawn out, it dissolves more salt, which moves more salt into the cheese, etc.
If your salt is really fine, like table salt, that movement will be too rapid and the salt gradient will get high very fast, which will slow down the movement of the salt. Similarly, if it's too coarse, it will not move fast enough. Usually, a flake size works well. Most of the flake salts I've seen are the same, so I would use any of them, measuring by weight.
For making brine, use the cheapest salt you can that still dissolves readily. Sometimes using huge salt hunks, it takes forever to dissolve, so smaller bits help with that.
for brine: by weight, up to you on size, whatever you can tolerate to wait
for curd: just about any "flake" size will do. table salt size is too fine.