How does one decide how to split up a story into parts if the book is too long and the story is continuing into another book? Is it OK to end a book with a cliffhanger, or should you always come to some kind of resolution?It is OK to end a book on a cliffhanger. You should just be aware that if buying your book requires buying two or more books, the publisher is going to weigh their enthusiasm against the higher investment required.
If you're starting a series, consider whether you're writing the type of book that often becomes a series. Genre fiction often carries series. Quiet, literary novels... not so much.