The current limitations of PostgreSQL's text search features are:
The length of each lexeme must be less than 2K bytes
The length of a tsvector (lexemes + positions) must be less than 1 megabyte
The number of lexemes must be less than 264
Position values in tsvector must be greater than 0 and no more than 16,383
The match distance in a <N> (FOLLOWED BY) tsquery operator cannot be more than 16,384
No more than 256 positions per lexeme
The number of nodes (lexemes + operators) in a tsquery must be less than 32,768
For comparison, the PostgreSQL 8.1 documentation contained 10,441 unique words, a total of 335,420 words, and the most frequent word "postgresql" was mentioned 6,127 times in 655 documents.
Another example — the PostgreSQL mailing list archives contained 910,989 unique words with 57,491,343 lexemes in 461,020 messages.
If you see anything in the documentation that is not correct, does not match your experience with the particular feature or requires further clarification, please use this form to report a documentation issue.