Scrivener's word-count facility make writing drabbles of exactly 100 words easy to write. YMMV, of course, and Scrivener is a mac only app, I am afraid. For once us mac users get something amazing that you non-Apple personal computer users do not! It is usually the other way around and windoze P.C. users get games and software that are not available on the mac (or *nix or any other operating system that is more stable than windoze).
Yeah - just sayin'. Just my £0.02p.