"제가 먹었던 피자는 컸어요."
Translation:The pizza I had eaten was big.
I agree with @wintertriangles about the nuance of remembering, having a flashback.
You can read more about it in (too much) detail on the howtostudykorean website.
In my KIIP class, we also saw this example to understand the difference between the present and past form of this pattern:
그 책은 제가 읽던 책이에요 --> you haven't finished reading yet, you are in the process of reading the book.
그 책은 제가 읽었던 책이에요 --> you finished reading that book.
Hard to understand but read this: