There's no space penalty or performance penalty, and they can be arbitrarily large.

I'd echo Mike's concerns about with/without time zone as well on the timestamps. You can always put "with timezone" on the selects, though, if you need to.

Re: Schemas: The hierarchy is server->database->schema->table. A connection for a particular database can only access that database. So if you want to segregate tables within a database, use a schema.