The great thing about the 260 is versatility. It works as a baselayer, it's great-looking casual outerwear, and it's an ideal mid-layer too. This shirt breathes well enough to be comfortable at 65° or so, but gets pretty toasty if it's warmer.
If you want the 260 to fit very snug as a dedicated baselayer, order your usual size. If you want more flexible use as I mentioned above, order a size larger. I am 6'5" and 250lbs, and wear an XL in almost every brand, but I much prefer the XXL Icebreaker stuff. The XL fits almost like a compression layer.
Great product - don't hesitate to pick a few up if you're undecided.