Student or Learner
Can I say ' I ate a small breakfast this morning." or only "had a small breakfast"?
Yes, but it doesn't mean 'opposite of big'.
If you say "I ate a little breakfast."', it means you didn't eat much. It's not quite the same as describing the size of your breakfast.
"Did you eat anything?"
"Yes, a little."
"Yes, I a had a little soup."