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
I should mention that I actually just tested this with gnu linux, not busybox. Looking at the busybox page, it does not look like du supports the --files0-from option.
You can change the above command to this to have it work on busybox:
du -ch $(find -type f -mtime +60) | tail -n1