The current limitations of PostgreSQL's text search features are:
The length of each lexeme must be less than 2 kilobytes
No more than 256 positions per lexeme
Position values in tsvector must be greater than 0 and no more than 16,383
The length of a tsvector's data (lexemes + positions) must be less than 1 megabyte
The length of a tsquery's data (lexemes only) must be less than 1 megabyte
The match distance in a < (FOLLOWED BY) N>tsquery operator cannot be more than 16,384
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.