I guess i'll go ahead and add my 2 cents.
I voted for 4 (combination of 1 & 2)
For me to consider a book "small", if it's an individual stand alone book, i would say the line would be drawn somewhere in the 300-500 page count.
If it's an entire series of books, i would consider it small if all of the books added up to probably less than 1000 pages in total, divided up over however many books.
I consider 600-800 pages in a single book to be a good-sized, mid range sort of read, and up to and over 1000 pages to be a long read.
Edit: also, i suppose it's important to distinguish, we're referring to novels, right, not like novellas? since novellas usually don't go more than like 100-200 pages. Because a novella is certainly a short book, and if we include those, I would say i would consider "short" a novella, and then medium would be novels up to 800 or so.