How to find all files that are larger than 1GB on the Mac HD?
You can use the -x to avoid traversing mounted drives:

-x      Prevent find from descending into directories that have a device number different than that of the file from which the descent
        began.


You command should then be

$ sudo find -x / -type f -size +1G


Regarding the fact about the missing documentation: is not missing is just not easy to find. At the end of the PRIMARIES section of the man page


  All primaries which take a numeric argument allow the number to be preceded by a plus sign ("+") or a minus sign ("-").  A preceding plus sign means "more than n", a preceding minus sign means "less than n" and neither means "exactly n".