Minecraft wiki entry for Managing_slimes_in_superflat_mode, associated text: This slime pit is 5×5×3, and has been dug down to bedrock and given a one-block 'lip'. The lip ensures slimes already in the pit don't sometimes push new arrivals away before they can fall in. Recommended size is about 7×7. 4×4 is about the minimum practical size because otherwise larger slimes arriving at an angle won't fall in. Rather than making one enormous pit, it's usually better to make several medium-sized pits in different areas.