“Trim Size” is simply the size of the finished book’s pages. The most popular size for POD at CreateSpace is 6×9. It’s a good size, but if you want something a little smaller, that’s also available. Lulu offers a size they call “pocketbook”, and that’s 4.25×6.88 inches … more commonly called a “paperback” by everyone else.
While there are many options, for a fiction book you should stick with something in the 5.25×8 to 6×9 range – it’s what your readers will expect for a fiction book, and it works well for the standard lengths of novels (and a bit longer, too.)
If you can, go to your local library with a ruler. Find books of each trim size you’re considering. Hold it. Open it.
Make your choice be something you like to hold. If you don’t actually like what you’ve created, nobody else will, either.