What does "doorstopper of a novel" mean?

I saw this phrase on the internet sites, but I don't understand what it means. I looked up "doorstopper" in the dictionary, but it doesn't make sense. If this is a figurative expression, please tell me what it means.

Here's my guess: the novel is not worth reading, its only use it as a doorstop.

I just "googled" it and it appears to be used to denote a very large book...so large that it could be used as a doorstop.

It's not a common description. I would take it to be big but not necessarily as worthless except as a doorstop.

It sounds analagous to 'blockbuster' but that of course means big and bestselling.

A good synonym is: a paperweight. It's good for holding down papers!