“Trim size” refers to the sizes of books. In fact there are charts that tell which sizes are “industry standard” and available in different formats. The availability from self-publishing platforms can affect your decision on where to upload, distribute, and publish your book. For example, CreateSpace does a great job for self-publishers but by design is meant to be more on the user-friendly side, so the amount of technical know-how you need is less but so are the publishing options on things like trim sizes, formats, and distribution, as compared to Ingram’s self-publishing platform, Lightning Spark, where you have many more professional options but a little more know-how is required. Each platform has a PDF that explains file preparation and lists trim sizes.
For example, speaker/author/trainer Erin Mahoney and I did a beautiful little book recently called Positive Vibes. We wanted a small format in cloth hardcover with a dust jacket. Ingram had it, but the smallest available was 5″ x 7″. That trim size was available in softcover with CreateSpace and we wanted to upload to both, so that’s what we went with. The Big Five publishers have access to more by way of printing, and produced Oprah’s little book in a size unavailable to self publishers. But that’s fine, Positive Vibes turned out beautifully!
I always check and try to decide on trim sizes before we get too far into a project. It helps, especially for cover design, and I’m anal-retentive, a bit. In fact I mock up the book file in the chosen trim size from the beginning (I loathe 8.5″ x 11″!). There are design benefits, and it’s more fun for me, which is important!
Certain trim sizes will be expected as genre-specific. Non-fiction books are very often 6″ wide by 9″ high. Textbooks are often letter-sized (8.5″ x 11″). Books of poetry, fables, and small business books are often 5″ x 7″, and if a business or philosophic title, exoected to be pithy at that size. Thicknesses are also expected as per genre.
The best exercise I know of is simply going into a book store once in a while and picking books off the shelves. You’ll want to be familiar with reader expectations, to meet them in ways, and exceed them in others.