I do 21 blocks and have almost no deaths. I think you can use 23 blocks but some die. I usually excavate three blocks above and three blocks below the mob spawner cage. Mobs can spawn around a spawner in the air so if they spawn at the top of the ceiling then they have further to fall. Some mobs spawners are too close to bedrock to get 21 blocks of fall so instead of using a water elevator I keep a chest with stone swords nearby of use a lava blade or something to hurt kill them.
Stay fluffy~