Trim size refers to the final physical size of the book once it has been printed and bound. It is presented as two figures such as 8 x 10 inches with the first figure always being the width followed by the height. Books are printed on larger sheets of paper and trimmed to the correct size after they have been assembled ensuring a neat professional presentation with a clean cut for every page.
Although there are a multitude of possible trim sizes from which to choose a convention related to trim size has developed over the years and is ignored at the peril of the author. The most important consideration is to create the ideal reading environment for the reader, which will vary according to whether the work is fiction or non-fiction.
Fiction is invariably read for pleasure often in a relaxed manner perhaps slumped in a favourite armchair, lying in bed or maybe while travelling. Novels are typically 5 x 8 or 6 x 9 inches because that is relatively easy to hold in one hand. Fiction is invariably continuous unbroken text written to be read without break for a relatively long period. Research shows that long lines are more tiring to read and that the maximum line length should be 100 mm (4”) which would give a book about 150 mm (6”) wide.
There is always a temptation to be different, too stand out from the crowd but that is unlikely to lead to success. Libraries and bookstores are far less likely to stock a book that is a radically different size to others in a genre because they simply will not fit onto the shelves. Even if a reader does buy the book, they may well not enjoy the experience if the size is not reader friendly and would be unlikely to buy another book from the author.
Non-fiction tends to be published in a significantly larger trim size in part because the nature of the material requires diagrams and or photographs and they need to be easy on the eye. A minimum trim size of 7 x 10” or 8 x 10” allows technical diagrams to be large enough to be clearly understood. This would typically be the case with domestic DIY books and car manuals, which with the larger trim size can be opened flat for easier use. Reader comfort is not an issue with such texts because they are not designed to be read continuously for a long period.
When inserting illustrations into a book avoid the temptation to fill every space with texts and images. This makes for a less comfortable reading experience with the reader literally having less space in which to think. White space separating illustrations from both other illustrations and the text helps draw the eyes into the specific part of a page on which the reader wishes to concentrate. Obviously cramming as much as possible into a page reduces the number of pages and the cost of publishing but it will be a hard and uncomfortable read giving the impression of an old-fashioned stuffy text. A larger trim size allows more white space making for an airy lighter feel that is easy on the eye.