I think there is a simple way using the model previewer in BrawlBox. Just open the file .pac file which contain the capsule, press Ctrl+P or click preview on the model (.mdl0) file inside the main .pac file, and then you may change the size just by changing a bone's scale in the right panel.
Here is a brief tutorial for the Brawlbox model editor:
http://code.google.com/p/brawltools/wiki/ModelEditorTutorial