estimating table size

Given a table, foo, created in a database but not populated, is there a
procedure that will return an estimate of the size of a given tuple in
that table? It looks like pgstattuple reports on actual table pages; I'm
looking for something that reads the lengths of each row, and knows what
the storage overhead would be (extra 4 bytes for varchar, for instance).

Alternatively, is there documentation somewhere that can give me a good
rule of thumb for this? I can add up the lengths of the columns and make
my own judgements about averages for varchar columns. The "hidden"
storage overhead is what I'm wondering about, I guess.

Thanks.

- DAP
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David Parker Tazz Networks (401) 709-5130