infact the lionfish will not venture into the anemone, however he will patiently sit outside the anemone and wait for the clown to dart out for food-- at which point the clown will become lionfish food
Now if your discussing dwarf lionfish (zebra, dwarf fuzzy or fu man chu) and you have a good sized clownfish- then the rule of thumb is slightly larger than 1/2 the body of the lionfish is considered safe. (but no guarantees)