Question:
You can also take your shirts to a tailor (or a friend who can sew) to have them tapered. The "cheap" way is to just bunch the fabric in the back and sew two seams down, the "expensive" way is to redo the side seams. For reference, in NYC the first method cost me $20 at your standard dry cleaners, the second cost $35 at a full-service tailor.

Answer:
Sizes certainly vary from one store to the next. Shop around. But talking about the ones you already have, it's easy to make something smaller, take it in, but to stretch it, (just one opinion) would make it look cheap.

Answer:
My sister chops my shirts up for me. If my dad ever gets me larges, she takes in the side seams, the arm hole, and brings up the bottom hem. I'm spoiled.
- Rich

Answer:
Originally Posted by _R$_ My sister chops my shirts up for me. If my dad ever gets me larges, she takes in the side seams, the arm hole, and brings up the bottom hem. I'm spoiled.
- Rich You're not spoiled for enjoying a good fit. All dress shirts I buy from the store are usually too baggy for me in the stomach area. I usually get them altered to take the sides in as well as narrowing the arm openings. If I intend to wear the shirt untucked, I will also shorten the length of the bottom.

As a side note, Macy's sells Claiborne shirts labeled "Modern Fit" and they fit superbly without any alterations. Hugo Boss also sells "Slim Fit" shirts that also fit nice.