It seems that either one of these commands can be used:
sudo find / -size +1G
sudo find / -type f -size +1G
But when it gets to /Volumes, it actually look at all the back up files by Time Machine in the external hard drive. (and another hard drive for data).
I was going to cd to /Volumes/Macintosh HD and start the find there using find . instead of find /, but then I did an ls and there is a Volumes right there again.
Any option to use the Finder as well (the GUI)?
Another thing I found puzzling is that none of the manpage of find on Mac OS Lion or on the web mentioned the +1G... all they mentioned were the format 1G for exact match.

du– enl8enmentnow Apr 06 '20 at 17:54