Mudwader, I have the HSS Ultra Lite harness. I'm a little bigger than you and I went with the L/XL. It seems to have plenty of adjustment. Always better to have more than enough in this case. Also, if you need extra layers for colder weather, you can adjust the harness accordingly. I'm usually a little heavier in the fall and winter, so it's best to have the option to adjust to suit my midsection.
If the L/XL happens to not fit like you'd like it, return it to where you bought it and tell them it was a gift and the size is wrong.
Good luck this season and stay safe.
"He who knows nothing is closer to the truth than he whose mind is filled with falsehoods and errors." Thomas Jefferson