I've found Pressbooks are actually easier on the eye: because with the slightly larger page dimension, the type is slightly less cramped than in a typical trade paperback. (pressbooks.com)
From what I've seen, the most commonly used page dimensions for trade paperbacks are 6 ″ x 9 ″, 5 1/2 ″ x 8 1/2 ″, and 5 ″ x 8 ″. (jamigold.com)