Beside many anaerobic single cell organisms, there are some annelid worms that are obligate anaerobic at least in their early development stages. Probably, due to low concentration levels in nature, some of the other respiration methods (like uranium or iron reduction) can limit the maximum size an organism will grow. Given the lower energy level extracted compared to the oxygen respiration, what size limits enforces the sulfate, sulfur or methanogenesis respiration? What's the biggest discovered organism, extinct or alive, that is obligate anaerobic?
There are three worms which have been found in the sediment of the mediterranean seafloor, which not only live without oxygen but also do not tolerate the exposure to oxygen. They belong to the metazoans, for more details see either the report (reference 1) or the original article (reference 2). They reach a size og about 1mm.