This question is so subjective, for some it’s normal, for others it’s too big, and others again it’s too small and they’d eat even more stuff after it. Everyone differs,
The potato itself is not the issue, it’s what you put on it. And how much of it you put on it. In addition as others have said, they vary massively in terms of size.
More than that though, everyone’s actively levels and general appetites vary. Some people eat a lot more than they physically need, some folks under eat and some folks eat the right amount. Some folks don’t move much, some folks are moderat active and some folks are very physically active.
So you’re not going to have any decent singular answer here op.
For a moderately active person, who was a healthy weight, who didn’t snack between meals, and who ate a mid sized baked potato with a small healthy topping on it, then it would likely be the right sized lunch.
But as said, for everyone else it’s shades of grey, there is no one answer.