Size of mysql query result

[admin]

I'm building a web-based mail system, and want to find out how much space each user is taking up each time they login.
What is the best way to do this?

Perhaps:

size_sql = "SELECT * FROM pages WHERE user_name = $current_user_name";

$size_result = db_query($size_sql);

$current_size = mysql_field_len($size_result);


Or is there a better way?
Thanks