I need only drop the column and VACUUM FULL the table, and not the entire DB, right?
Do you seriously think that vacuuming the entire database is a possible requirement here?
In terms of estimated space savings, the data type documentation tells you how much space each data type consumes. A reasonable first estimate, knowing nothing else about the internals of the system, is to multiply that by the number of rows. That would provide a lower-bound estimate. If that amount makes the effort worth it then no further info is needed.