table size and storage location

Hi,
I have two questions:

1) how do I find out the size, in MB, of a particular table (called 'capture' in this case).
I tried entering the SQL query SELECT (pg_tablespace_size('capture'));
The result was the following:
ERROR:  tablespace "capture" does not exist

2) how do I find out where the actual files for the database are stored on my system?
Where is the default storage location?

Thanks!
Charlie