I know what you mean as I found the same problem when I started collecting. I'm about 6'1" and slim(ish) build. My suit jackets are 38" and most of my casual tees are size M. I find the following sizes work for me, but a lot of it depends on how you like to wear your NFL jerseys. I like them to be fairly baggy, but not hanging off me:
"Modern" Reeboks (last 2 years):
Replicas: Small;
Premier: Small or Medium (I find premiers to be neater then the replicas so can get away with either size);
Authentic: 46.
I have an Adidas authentic (I think from circa early 2000s) which is a 48 and gives me the same fit as a Reebok 46 authentic. I believe that Reebok authentics are larger than previous manufacturers (e.g. Wilson, Adidas), but perhaps someone can correct me if I'm wrong?
Were you after any particular jerseys? I am about to sell a small replica Browns (Winslow - 80) and a small premier Pats (Brady - 12) if you're interested in either of those? The Browns jersey is brand new (though I removed the tags) - never worn. The Pats jersey has been worn a few times but has no damage / signs of use.
As I've found (and Billsfan mentioned, above) you tend not to get too many reebok authentics in size 46, but you do see them now and again. I have 2 Chargers authentics - Tomlinson and Cromartie.