I've been in the same boat as you - set myself an amount of books to read that year, but then find myself searching for short books and avoiding long ones.

I've also tried planning out the books I want to read over a set amount of time, which works better in a way because then I can choose long and short books and adjust the time limit. The bad thing about that system is that often il see a new book that I either have to read for a course, or desperately want to read, and then the system goes out the window because it isn't in the plan.

It would be possible to do a number of pages in a set time goal, but I've never tried that