The sentence you wrote is almost correct, but it can be made a little better.
Original sentence: Elephants are so big, they drink more than 5 buckets of water a day.
The problem is that it's a run-on sentence because it joins two ideas with just a comma. Instead, we should use because to show why or separate them properly.
Here are two simple correct versions:
- "Elephants are so big because they drink more than 5 buckets of water a day." (This tells why they are so big.)
- "Elephants are so big that they drink more than 5 buckets of water a day." (This shows how being big relates to drinking a lot of water.)
Or, you can use two separate sentences:
"Elephants are so big. They drink more than 5 buckets of water a day."
All of these are easy to understand and grammatically correct!