Formula field size and compile limits
Key Takeaway
A formula that is too long and a formula that compiles too big need different fixes. Moving parts into other formula fields fixes the 3,900-character limit, but it does not fix the 15,000-byte compile size limit.
Formula fields have two kinds of size limits. One limits how long the syntax you type can be, and the other limits how big the database query compiled from that syntax can be. Each limit has its own fix, so knowing which one you hit tells you what to do.
A formula has two sizes
Think of a formula as an iceberg. The visible part stands for the syntax you type in the formula editor. The hidden part stands for the database query compiled from that syntax.
This compiled query can be much larger than the syntax, because it pulls data from every field involved, runs the operations, and often checks the results for errors. The size of this query is the formula's compile size.