The sentence is correct and usable in written English. You can use this sentence either in response to a statement (e.g., "You don't believe me?" "I meant what I said.") or when emphasizing the importance and sincerity of something (e.g., "I can't stress enough how much I mean what I said.").
I meant what I said, though.
"I meant what I said when I left.
I meant what I said Now go, great bird!
"I can't prove that I meant what I said.
I mean what I said, it's still how I feel.
I mean what I say.
I say what I mean, and I mean what I say".
I love the desktop app, it’s always running on my Mac. Ludwig is the best English buddy, it answers my 100 queries per day and stays cool.
Cristina Valenza
Retail Lead Linguist @ Apple Inc.