The jump into chapter books goes better when the first one is over before dinner. A short book teaches the shape of a longer story: a problem, a few tries, a last page. Then they are willing to start another.
Nate the Great and Hi! Fly Guy are the quickest. Mr. Putter and Frog and Toad Together are calm. Ivy and Bean and Junie B. Jones are noisier, with friends and trouble and a narrator who sounds like a kid talking fast.
Let them pick between two. A mystery feels different from a school story, and the wrong mood will make a short book feel long.
If they finish before you expected, that is the result you wanted. Put the next book where they can reach it, and do not immediately hand them something twice as thick.
When the last page comes, let them tell you the story in their own words before you reach for another book. If they can say what happened, the length was right.