When choosing a trim size be sure to understand the layout you are looking to achieve (vertical vs horizontal) and what that translates into a trim size.
I knew that I wanted a wide format for my first Angria book and was happy to see that Amazon KDP offered 8.5x11. I happily went on my way creating the book only to discover after it was complete that 8.5x11 (a vertical trim) is NOT the same thing as 11x8.5 (a horizontal trim). This held up the release of the book while I figured out what to do next. The largest Amazon KDP trim size is 8.25x8.25 which would have meant updating my book format to square. This meant a lot of work and probably lost content. Besides...I was so pleased with how the book turned out!
Thankfully IngramSpark trim sizes do offer 11x8.5 so all wasn't completely lost. However, this led to a decent amount of reformatting the book to meet the file creation guidelines for IngramSpark as they aren't quite the same as Amazon KDP.