When Picking Something to Read, Try the Reading Snowball
When I have a choice of several things to read, I often choose the shortest book first. I adapted this idea from Dave Ramsey's debt snowball.
The reading snowball means that you choose the shortest book in your list first and then move on to the next shortest book until you go through the whole stack, shelf, or list. This is a psychological trick that I use to help me feel like I'm making good progress through a list or shelf of books. This keeps me from feeling overwhelmed which the books that I plan to read.
I read a 700 page commentary and realized that I had neglected a lot of other reading while I was going through that one. I had a list of three commentaries that I wanted to read next, and started with the short one.
I don't always use this approach. Sometimes, you should read the most relevant or important book. For example, I'm starting a 700 page biography of Abraham Lincoln tonight. I'm also reading The Lord of the Rings right now. Those books are important to me. I'm not afraid of reading long books, but I am starting a 100 page book for my work reading too. That one was picked because it was the shortest book on my shelf of books to read in my office.
I don't think you should prioritize short books over long books. When I have a choice, though, and it doesn't matter which book I read next, then I choose them in order of shortest to longest.