Longdescription is managed in a separate table for all applications and is an internal object where you can't make change. In DB it is CLOB/BLOB type which doesn't have size associated with it.
If you want to decrease the size of the Long description box for users then You can set a height and width (in pixels) on the longdescription control in the longdesc_dialog in the library system XML which will be applied on all the applications.