Is it safe to expand this Macintosh HD partition?
Is it safe for me to try to expand the partition back to it's original size?


Yes, it should be safe to try to expand the partition back to its size. You can do this either by dragging the line or typing a number larger than 251 in the "Size" box.


  Do I risk losing data?


Any time you make changes to a drive (including resizing a partition), you risk losing data. I would argue that the risk is small but you should ALWAYS have an up-to-date backup. Make a backup immediately before you attempt the resize.