My tomato pair roam the full extent of the 170 gal. And they are the bosses. However, their main area is a sq. ft or so around the
bubble coral they live in.
The 40 would be iffy for them, but perhaps enough if they were alone or with something like a dottyback or damsel only.
For sure percs, who are bad swimmers and smaller would be better.
Orange skunks are also great for that size, and dont roam far from their home. As for size, I have a lone
pink skunk in my 180 that is much larger than momma tomato, she is huge.