If Bedrock, then maybe just 20 blocks and set up a trident killer at the bottom. If Java and you want them to die, use the formula some one put in the comments to calculate how far they have to drop, then add 3 for good measure. But if you want the do, drop them about 20 blocks and hit their legs. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.