Found it documented in Horizon source code:
// Operations within the stellar network have a total order, expressed by three
// pieces of information: the ledger sequence the operation was validated in,
// the order which the operation's containing transaction was applied in
// that ledger, and the index of the operation within that parent transaction.
Horizon response contains navigation links. Use them to retrieve next or previous results page.
Serving huge batches form the server is usually discouraged as both a client and server spend to much resources for results serialization/deserialization.
It looks like your Postgres server is either not installed or mis-configured. Can you connect to it from the command line?
You need to configure it to allow local connections. See https://www.postgresql.org/docs/9.1/auth-pg-hba-conf.html for more info.
As mentioned in the comments, this number is hard-coded in Horizon so can't be changed without recompiling. What's your use case for wanting to change it? If you run your own Horizon instance you can remove rate limiting and make as many requests as you like.