I wasn't able to find any information on the max length for a string data type in MongoDB on the main mongodb.org site. I'm coming from a relational database background and there are usually max lengths.
This other question should answer your question:
Documents larger than 4MB (when converted to BSON) cannot be saved to the database. This is a somewhat arbitrary limit (and may be raised in the future); it is mostly to prevent bad schema design and ensure consistent performance.
Note that in the picked answer for that question the commenter mentions that it could be as large as 8mb or 16mb now.
So to answer your question, it's not necessarily that a single string can only be a certain length, but rather that the whole document must be under 16MB.