To summarize @larowlan, @VMTrooper, and @vahid chakoshy solutions:
#!/usr/bin/env bash
if [ "$#" -eq 2 ]; then
echo "$(echo "scale=2; $(curl https://api.github.com/repos/$1/$2 2>/dev/null \
| grep size | head -1 | tr -dc '[:digit:]') / 1024" | bc)MB"
elif [ "$#" -eq 3 ] && [ "$1" == "-z" ]; then
# For some reason Content-Length header is returned only on second try
curl -I https://codeload.github.com/$2/$3/zip/master &>/dev/null
echo "$(echo "scale=2; $(curl -I https://codeload.github.com/$2/$3/zip/master \
2>/dev/null | grep Content-Length | cut -d' ' -f2 | tr -d '\r') / 1024 / 1024" \
| bc)MB"
else
printf "Usage: $(basename $0) [-z] OWNER REPO\n\n"
printf "Get github repository size or, optionally [-z], the size of the zipped\n"
printf "master branch (`Download ZIP` link on repo page).\n"
exit 1
fi