Through the TAP interface the query is limited to around 8kb long (or 8096 characters) on Firefox and 2kb on IE. A query directly to PostgreSQL is limited by 1 gigabyte - 1 character in size.
Although there are reports of PostgreSQL being able to join 1 million tables (it required patching and special configuration), we recommend keeping the number of joins to a minimum.
Then length of the resultset is not limited by any "governor"; however, the smaller the resultset the faster it will transfer over the network, thus copious use of the LIMIT clause is highly recommended.