I cannot understand this answer. I don't know much about biology or have a lot of trouble recalling stuff from a university biology book I bought for a course I never took because it was so long ago and it was confusing even then. I think I once learned that salmon can live in salt water or fresh water. According to https://fishcostarica.org/this-fish-lives-over-60-years/, tarpon can also live in fresh water or salt water. It seems to me that for tiny fish, they deed a different type of body with more solutes inside the cells in order to live in salt water and the tiny fish that can live in fresh water have a different type of body with fewer solutes inside the cells. And advantageous traits come with costly traits so a small fish that could do both would have more of a costly trait and therefore be outcompeted to extinction. There are plenty of fish in fresh water already eating the resources in the fresh water and plenty of fish eating the resources in salt water. And being able to move back and forth between them doesn't really enable a small fish to get more food. I think a big fish on the other hand would have a much slower rate of osmosis so it can easily pump water our as fast as it is getting absorbed into the cells or in as fast as it is getting absorbed out of the cells. Salmon also have some reason to go back and forth between fresh water and salt water.
Mollies , there are a few kinds , many are 2 to 3 " , live in fresh or salt . They seem to prefer fresh but often live in sea coast areas where tides ,etc. may cause intermittent changes between fresh and salt ( and brackish of course) . I used black mollies to start a salt water tank, I had a couple in the salt for months because it was not easy to catch them in a large tank. I expect there are other small fish such as puffers. The mollies adapt very quickly , just drop them in whichever water and they behave normally.
I didnt read beyond the paragraph about your study books situation, Euryhaline on wiki reveals tiny euryhaline fish... 4-5 cm Fundulus heteroclitus which presumably can withstand salt changes as juveniles, 2-3cm. Probably there are species a lot smaller because individual cells perform the osmotic rebalance, and cells are much bigger thans water and salt, so a smaller fish can just develop more of apparatus for osmotic balancing.