AandA, if you normally wear your pants (not belt) tight, you will need one size (2") bigger to accommodate a full-size IWB holster. If you normally wear pants that are not fitted and have just a little room, you only need to loosen your belt.
Please consider a stiffened gun belt and a professionally fitted holster. This is roughly $100-175 and will mean a WORLD of difference in how comfortable your carry is on an all-day basis. Holster can be Kydex or leather but should be fitted to your gun and have positive retention. You don't want to be in a bathroom stall and watch your gun skitter across the floor and out of the stall. Didn't happen to me, but someone I know experienced this nightmare with a generic nylon holster and a room full of men he had to explain it to.
“Malo periculosam, libertatem quam quietam servitutem." (I prefer the tumult of liberty to the quiet of servitude.). Thomas Jefferson
"All that is necessary for the triumph of evil is that good men do nothing."
-Edmund Burke, Loosely translated from Thoughts on the Cause of Present Discontents. (1770)