well i believe that the senegal bichir gets up to a foot. and to not get eaten by an oscar. well just eyeballing from videos ive seen id say maybe 6 inches b4 theyre wide enough to not fit into an oscars mouth. if your worried do what im gonna do, just get the bichirs, let them grow out, and then when they look big enough for the oscars at the store, get the oscars. im not entirely sure how to exactly answer your question but thats what im gonna do just to make sure theres no problems. now if you already have an oscar, try to get a big bichir.