GIMP Select Bounding Box Plugin
gimptoix-bounding-box is a GIMP plugin to select the bounding box
from the current selection
Download the zip file and extract it to the gimp plug-ins folder
On *nix OSes it might be required to chmod +x gimptoix-bounding-box.py
This plugin adds a menu entry at the bottom of Select Menu named ‘Bounding Box’