You're image size really depends on how much size you want to save I guess, although also if you're willing to have some non-visible pixel lose you could convert them as well. You can start playing around with 2048x1536 or similar just keep in mind your aspect ratio.
Here's the for loop I use for creating thumbnails, use as needed:
for i in `find "$LOCDIR" -iname "*hqw*"`
do
echo "$i"
TN=`echo "$i" | sed 's/hqw/tn/'`
echo "creating thumbmail for "$i" ..."
/usr/bin/convert -resize 200x200 "$i" -background none -gravity center "$TN"
done