code:

#!/usr/bin/sh

# No user name provided ...
_f(){ >&2 echo "[largestfilefor] No user provided."; exit 1; };
[[ -z $1 ]] && _f;

# Else, ...
USER=$1;
FILES=$(find / -type f -user $USER 2> /dev/null);
if [[ -z $FILES ]]; then
    >&2 echo "[largestfilefor] Found no file for the user $USER.";
    exit 1;
fi;
echo $(echo $FILES|xargs stat --format="%s %n" 2> /dev/null|sort -nr|head -n 1|cut -d " " -f 2);