For a paperback book, pick a book protector from the list of available sizes whose stated height is as close as possible to the measured height of your book.
It is OK for the book protector to be slightly larger (up to 4mm) than the measured height of your book but it must not be any smaller or it will not fit.
For hardback books the procedure is the same but add 2mm to the measured height of your book (to account for the thickness of the hardback covers).
When measuring the total width (front cover + spine + back cover)accuracy is not so critical because the book protectors are adjustable in this direction. Provided that the measured total width of your book does not exceed the width stated for the book protector, then you can be sure that it can be adjusted to fit.