Hi Andy,
Your pattern should have all the sizes (S - XL) on it - most patterns do. Each size has a slightly different line (dashed, dotted, etc) so you should be able to figure out which one is the Medium. You might want to trace it onto tracing paper to preserve the original pattern.
In case you do want to adjust it, pattern adjustments are not super hard but they’re also not as easy as you might think. A lot of adjustments at one area affect other parts of the pattern and the order in which you do it matters. Corresponding seam lengths have to match up.
There are a lot of good books out there, as well as info on the web. I liked this book quite a bit:
https://www.amazon.com/Pattern-Fitting-Confidence-Nancy-Zieman/dp/0896895742
Thru-Hiker kits are great and the instructions are excellent. Still, if you’ve never sewn before, expect a bit of a learning curve, especially dealing with insulation bulk. Most people make at least one “practice run” with muslin cotton first, and I also actually made a Kinsman with cheap ripstop and insulation before I tried using better materials. It was worth the time to do so.
Here’s a few shots:
https://www.flickr.com/photos/97152865@N05/8990842326/in/album-72157634009876955/
Good luck and have fun! I made multiple Kinsmans and each one got a little better. The difference between my first one and my last one was considerable!
