I started with a single pair of the micro modal ultra soft in L. I'm about 5' 10" 220 lbs. with a slight belly - guy next door type. 38" waist.
The underwear is VERY comfortable, but the rise in this underwear is such that I have to fold the waistband over on itself for them to fit right and "operate" correctly. Admittedly, I'm a grower, not a shower, and I have no complaints about how they fit when I do this maneuver.
Intrigued, I decided to try the Bamboo Sports Trunks because they looked like they had a lower rise. I ordered a 3Pack in L and for the hell of it, another pair of the Micro Modal, in M this time.
Again, the Bamboo is very comfortable, but yet the rise is STILL to high to realize any benefit from the design of this product. The Micro Modal in M definitely fits better than the L.
So now I have 5 pairs of underwear, only 1 pair of which fits me well. My advice to someone with an average build is to order one size down. I'm hoping someone at Separatec will contact me to remedy the abundance of rather large underwear I now own!