There are lots of ways to list files and sort by their size with the help of pipe operator, for example:
ls -l | sort -n -k 5
But ls command already has builtin support for this:
ls --sort=size -l
you can reverse the list with -r option:
ls --sort=size -lr
or you may add -h (human-readable) parameter for good looking:
ls --sort=size -lrh