We recommend you stick with industry standard sizes for books and choose one from the following list:

Books in Black & White (color cover), Standard Sizes

5 x 8 inches, 12.7 x 20.32 centimeters
5.06 x 7.81 inches, 12.9 x 19.8 centimeters
5.25 x 8 inches, 13.335 x 20.32 centimeters
5.5 x 8.5 inches, 13.97 x 21.59 centimeters
6 x 9 inches, 15.24 x 22.86 centimeters
6.14 x 9.21 inches, 15.6 x 23.4 centimeters
6.69 x 9.61 inches, 17 x 24.4 centimeters
7 x 10 inches, 17.78 x 25.4 centimeters
7.44 x 9.69 inches, 18.9 x 24.6 centimeters
7.5 x 9.25 inches, 19.1 x 23.5 centimeters
8 x 10 inches, 20.32 x 25.4 centimeters
8.5 x 11 inches, 21.59 x 27.94 centimeters

Books in Color (full color), Standard Sizes

5.5 x 8.5 inches, 13.97 x 21.59 centimeters
6 x 9 inches, 15.24 x 22.86 centimeters
6.14 x 9.21 inches, 15.6 x 23.4 centimeters
7 x 10 inches, 17.78 x 25.4 centimeters
8 x 10 inches, 20.32 x 25.4 centimeters
8.5 x 8.5 inches, 21.59 x 21.59 centimeters
8.5 x 11 inches, 21.59 x 27.94 centimeters

Many other non-standard industry sizes are available but may create problems with marketing at stores that only accept Industry Standard Sizes.