Use Of Large Field Definitions For Small Values

Hi
This is a question of "what does it cost me".

Lets say I have an integer value which would fit into a smallint field
but the field is actually defined as int or even larger as bigint.
What would that "cost" me ? How would definitions larger than I need for
the values in the field affect me ?
Its obvious that the volume of the database would grow but with the size
of resources etc that we have nowadays disc space isn't a problem like
it used to be and i/o is much faster and many people would tell me "who
cares" , or IS it a problem ?
How does it affect performance of data retrieves ? Searches ? Updates
and inserts ? How would it affect all db access if tables are pointing
at each other with foreign keys ?

Thanks !

David Greenberg