So now that I have my 1997 Colt Sporter A2 HBAR the only change that I want to make (or would make) is to maybe get a better trigger.  This is a stock rifle from what I can tell; police turn-in.  

I can't wrap my brain around the pin size thing though.  Can I shop off-the-shelf triggers (like Geissele, LaRue, etc) or do I need to be looking for something special?  The LaRue page mentions "Small Pin Lower Receivers .154"

Short of digging through the moving boxes to find my caliper, does anyone know offhand if these will work, and explain the pin size issues?

Geissele has a page that explains all of this and how to tell, many of the Colt guns between 1991 and 2009 run a good chance of being large pin size.

https://geissele.com/colt-rifle-users-guide/

Small is 0.1540, large is 0.1690, if confused measure your pins.