I've only recently been consciously aiming for reasonable portions. Prior to that, I ate as much as I fancied of anything, as long as it fit within my kcal budget.

I found something on-line that listed what actually constitutes a serving, but offhand I can't recall where it was.

You'll find lots of sites if you type "serving sizes" into a search engine. (There may even be something on this one.

)
I try to stick to within these guidelines, bearing in mind that it's okay to have a couple of portions as part of one meal, e.g., 2 slices bread. I wait until 20 minutes or so after I've finished eating to decide if I
need more, and usually I don't. I also find it useful to brush my teeth immediately after I finish to discourage getting seconds for the sake of it. Mouthwash is my friend!
Doing this has made me appreciate just how much I was over-indulging before.

I wouldn't say I eat much less overall, but my diet is definitely more balanced, and I'm not getting that awful stuffed to the gunwales feeling after every huge meal. Less food, more often is working for me at the moment.