As others have said, the small format paperback 「文庫本」is common for light novels, though the vast majority of literature can also be found in this format.
Some light novels are printed in other forms too, like my copy of 本好きの下克上, which is larger even than the common manga book, and closer in size to the second commonly available print format of literature, 「単行本」, which is usually hardcover and quite a bit larger.
See https://www.mottainaihonpo.com/kaitori/contents/cat07/020-bunko-toha.html for more (where they also talk about another paperback format 新書).