Get size of variable-length attribute as stored on disk

[Christophe Pettus]

Hi,

Is there a way from SQL to get the size of a particular attribute (field in a 
row) with a variable-length typed as stored on disk (including compression)?  
The use case is to determine the size distribution for setting 
TOAST_TUPLE_TARGET and making other decisions about what to store in that field.

Thanks!