I have to say that I've got a towel rail as the only source of heat in my bathroom. The bathroom isn't massive, (approx 3.6m x 1.5m) but I put a 1200 x 600 rail, as they don't output as much heat as radiators, so you generally have to go a bit larger. Plus added to that you generally cover the rail with towels, which abosrb the heat, as Dan has said.
My bathroom is always nice and warm, so you can do it, just take into account the heat loss from the room. Nice heat loss calculator here: