...kaj eliris triope.
What is the difference between "big" and "large" ?
No real difference (though in some contexts and phrases, one might be more common than the other). Both are perfect translations of granda.
I have reported it but Duo did not accept my use of large.
Good that you reported it.
Isn't a "boat" a small thing, and a "ship" a large thing? So wouldn't "ŝipo" be better here?
If this is a reference to the biblical story of Noah, then maybe even "arkeo" would be better yet? (I got "arkeo" from lernu, in case there's a better word).