Calculate storage size of rows?
You can use pg_column_size() to get the size of a complete row, not only for a single column.

So to find the size of all rows for a specific customer, you can use something like this:

select sum(pg_column_size(t) + 24) 
from the_table t
where customer_id = 42;


(The 24 is the storage overhead per row in a table)

You would need to do that for all tables you are interested in. 

This does however not include the size of those rows in the indexes.