Does anyone know what kind of screw size (diameter) has the Joby Ballhead 1k https://joby.com/ballhead-1k on the BOTTOM of it? Same diameter as the top screw, or larger diameter? I can't find any picture of it from the bottom nor any information about it in the specs.
If you click on the Link of the Gorillapod, that the ballhead is designed for, you'll find, that it has a 1/4" standard mount.
Therefor, yes. The bottom is the same as the top
The product page for the ballhead states:
Evolved Design: Removable mini ball head can be used on GorillaPod 1K or standard tripod with ¼"-20 mount.
Therefore the threaded socket is ¼"-20.