We talked about the craft and structural differences between short fiction and novels -- number of plot arcs, density of try/fails, how many character arcs can be accomodated, the value of economy versus expansion (or yes, even digression). I drew her a series of little charts, based on something K.W. Jeter had shown me years ago. Lots of discussion about what made a novel a novel.
But as for idea sizing? It's not the size of the idea, it's what you do with it. Yes, big ideas rolling across large landscapes with a cast of thousands can fit quite well into novel-sized manuscripts. Think epic fantasy or tech thrillers, for example. On the other hand, Proust wrote an entire novel about dunking a pastry in coffee.
But for those of us who come in from the short fiction side, including both the evanescent mkhobson and myself, there's a special trap. Short fiction rewards economy of prose. Not just economy, but a specific sort of clever pithiness. It's habit of mind and writing which can be very powerful in short fiction, but at times downright counterproductive in novels.
The confusing thing is that both novels and short fiction use almost exactly the same toolbox, with many of the same techniques, to produce two different results. I've made that comment before here on this LJ -- when you've trained as a cabinetmaker, framing carpentry doesn't make all the sense in the world.
So how do you know when an idea is novel-sized? When it wants to be written long. The approach is key here, whatever the idea. Contrast Dame Anna Russell's Ring der Nibelung [ Amazon ] with Wagner's [ Amazon ].
A novel is not a destination, it's a journey.