Pants sizes are plain and simple (for men). There are various waist sizes and then length sizes; all of the sizes are in inches.
For shirts and athletic pants, you have just of sizes of small, medium, large, x-large, and so on. For the past 5-6 years, I have been in the weird transition size between small and medium called smedium. Whenever I am asked what size I am, I have the arduous decision to choose between small and medium. But this is after attempting to get more information about the shirt: Is it Unisex? Do you know what brand shirt it is? Will it shrink in the wash? Though, majority of the time, none of the answers are available.
Here is J. Crew's Sizing Chart:
My neck is barely 15", my arm length is exactly 33", and my chest is 38" on the biggest of days. I can decide whether to wear a form fitting (read: tight) shirt or a shirt that looks nearly a size too big. And due to American's obesity problem, particularly old men, shirts that are not categorized as "fitted"/"slim fit" don't fit due to the enlarged stomach/gut area. I wonder if other nations are the same way, or do they have a fit for larger men...
Life was so simple just being a plain old small.