Hey Chris, I have two mediums that I'm taking to Denali for clean and dirty clothes. They seem to fit everything I need for an extended expedition (clothing wise) without any problems.

It's difficult to determine the proper size without knowing the size and material of the clothing you wish to compress. The easiest way to determine this yourself is to take all the items you wish to compress and place them in a bag and slightly compress the bag (a plastic grocery bag works great for this) until you get a rough square. Take the measurements H x W x D and multiply them together to get the volume in cubic inches (or centimeters). Next, figure out the closest cubic inches (or cm) of the compression sack. Since the sack is a cylinder, you can either use an online volume calculator or do it yourself (I've provided the cubic inches for these sacks below). To figure the volume of the bag, take the radius (typically 1/2 of the smaller measurement) multiply that number by itself then multiply that number by 3.141 and then multiply that by the height of the cylinder, which is typically the larger measurement. So, for the smallest sack here (XS), the dimensions are 6x14in, which = ((3*3)*3.141)*14 = 396 cubic inches. So, after you know the cubic inches of your gear, match that to the cubic inches of the sack. Say your very slightly compressed gear takes up approx. 790 cubic inches, you could go with a small or a medium. I generally go a size larger (medium, in this instance) because I am too lazy to pre-compress my stuff into the sack. XS = 396 in^3 S = 616 in^3 M = 904 in^3 L = 1272 in^3 XL = 1991 in^3