I find it hard to write flash fiction, although I find that most stories determine their own length and so sometimes end up with flash fiction. Writing to a fixed length is incredibly difficult for me and I find the minute I start doing that I lose something in the story. So I tend to just go with the flow unless it's for an assignment or competition.

I think I prefer writing short stories than novellas/novels but mainly because I don't have to worry about thinking in great detail about plot arcs, etc, and I'm too lazy to write longer pieces and rarely finish them. But at the same time I'm not such a fan of reading short stories (although I've become more interested in reading them as I've been writing more)