A book’s trim size is simply the size of the book. In the US, trim sizes are expressed in inches; the first number is the width and the second number is the height. Books come in an array of trim sizes, the most common being 5 x 8, 6 x 9, 8 x 10, and 8½ x 11. While a portrait (width smaller than height) orientation is more common, books can also be landscape-oriented (width larger than height). However, landscape-oriented books are often more expensive to produce than portrait-oriented books (i.e., a trim size of 10 x 8 is more expensive to produce than a trim size of 8 x 10).
Choosing a trim size will depend on personal preference, the trim sizes your printer/publisher can produce, the number of pages in your book, how your book will be used, book content, cost, and various other factors.