I am 5' 8" tall, weigh about 170-175 lb., and wear a size 44 chest jacket and 34 waist pants.
I carry IWB at 3:30. This position is far enough back to take advantage of the hollow in my hip but not so far back that I get excessive printing when I bend at the waist. In this position, a Glock 26 disappears under a polo shirt. A Glock 19 will conceal, but I need to be more careful with shirt selection as well as bending at the waist. A Glock 19 will also eventually wear holes in my shirt, unlike the 26. Under a loose fitting sweater, either will conceal quite well, but under some of my tighter fitting sweater vests, the 19 becomes more difficult to conceal. Magazines for the 26 are the OEM 10 round magazines. I have tried +2 base pads, but did not like the result, and decided that the 10 round magazines best suit the purpose of this gun.
I do not buy my shirts oversize, but do not buy them skin tight either.
Current holsters for the 26 are an old SideArmor holster with Blade-Tech pull the dot belt loops, resulting in zero cant. For the 19, I have used a Blade-Tech IWB with FBI cant, but recently added a Streamlight TLR-7 and JMCK IWB #3 with 10 degree cant and widely spaced pull the dot loops.
In the past, I have carried the 26 much more often than the 19, but lately I carry a Sig P365 most of the time, using either flat base 10 round magazines for pocket carry or 12 round magazines for IWB carry. I am currently using a JMCK IWB #3 with a 10 degree cant and widely spaced pull the dot loops. This gun and IWB holster combination splits the difference between my 19 and 26 while being thinner than both.