Employ the magic of "backticks" for this one.
First of all, the command for finding all files that belong to a certain user is:
"find ./ -user mette -print"
For each of these files, we want to show the size, the command for that is:
"du -h"
To run that command for each file, use backticks like this:
"du -h `find ./ -user mette -print`"
Note the special backticks, normal single quotes (') or apostrophes (´) will NOT work.