When 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":"TENANT_FTS_INDEX_TOO_LARGE","parameters":[],"reason":"Bad Request"}
The message could be clearer.
The limitations are described here: https://docs.atlas.mongodb.com/reference/atlas-search/limitations/