Try pipe the output of find to du and specify the --files0-from - flag:
find -type f -mtime +60 -print0 | du -shc --files0-from -
This should give you a grand total at the end
To get just the total, pipe that output to tail -n1:
find -type f -mtime +60 -print0 | du -shc --files0-from - | tail -n1