Questions about PostgreSQL backup/restores
2) pg_dumpall is what you're looking for if you want all objects (globals, roles, etc.). 

1) You noted that it is subjective. On my dev laptop (Win7 x64 .. i7 4 @ 2.2 ... 12 GB RAM ... normal, spinny disk @ 5400) I backup a 4GB pg cluster in about 15 seconds. I don't know if it scales linearly. 

Finally, depending on your OS, I'd verify that it's not actually being backed up. I've seen pg on linux being backed up at the file system level using rsync. Without getting into a debate about the relative merits of these differing methods, I'd verify that this isn't already implemented.