Does removing duplicates change the order of my list?
No. The first occurrence of each line stays where it was and later copies are removed, so the original order is preserved. Use Sort Lines afterwards if you want it alphabetized.
How do I treat "Apple" and "apple " as the same line?
Tick "Ignore leading/trailing spaces" and "Ignore case". The first version of the line is kept exactly as you typed it.
Is there a size limit?
Everything runs in your browser, so tens of thousands of lines are handled instantly on a normal laptop or phone. Nothing is uploaded.