This chart should serve for GI stuff.  Non-GI copies, I dunno.  Propper makes GI stuff, so they should hold to the size chart.

I'm assuming that by "coat", you mean the typical GI shirt, not the GI Field jacket.  No matter; the sizing chart explains all that.  The Field jacket is indeed considerably larger than civvy counterparts, in order to facilitate layers being worn under the field jacket.  IIRC, the field jacket is no longer a part of the modern GI uniform, and IMHO that is too bad.  YMMV.

http://www.armystudyguide.com/content/army_board_study_guide_topics/uniforms/army-combat-uniform-acu-s.shtml

It's my understanding that most GI pants are a bit weak in the crotch area, and so one should, perhaps. order one waist size up, so as to give a little more room down there, as a matter of course.  FWIW, most competitive shooters who are required to assume sitting or kneeling shooting positions wear pants significantly larger in the waist than street pants, and very often use suspenders in lieu of belt.

I strongly suggest that you consider visiting a surplus store that has actual GI uniforms available for in-store try-out.  It is true that most GI uniforms are sized a bit on the large size, but there is no substitute for trying-on an article of clothing.  Most GI uniforms, being of a nylon/cotton blend, will show a little bit of shrinkage after being placed in a dryer, so be aware of that.  Such shrinkage will also happen eventually, no matter what.

As a general rule, wearing an item of GI clothing that is one size up from your ideal size gives the user very minor problems, but one size too small and one comes to hate the item very soon.  I suggest that you try out some GI stuff, and don't compromise on proper sizing for YOUR body.