hen you create an index definition on the free plan that exceeds 3KB you get the following message:

{"detail":"A tenant FTS index may not exceed 3 KB in size.","error":400,"errorCode":"TENANTFTSINDEXTOOLARGE","parameters":[],"reason":"Bad Request"}

The message could be clearer. I had no idea what it meant and had to search the docs for that.

The limitations are described here: https://docs.atlas.mongodb.com/reference/atlas-search/limitations/