I Like the Thread name "What size to conceal" My answer, Any size you need to conceal!
But seriously folks, Basically you can conceal anything depending on manner of dress, type of holster and body size.
Most good "GOOD" holsters will conceal a full size gun so long as your choice of shirt hangs correctly.
Lets face it, people are not looking at you that closely, they have other things to worry about.
LEO's, good security personnel and other members of the carry community may notice that slight to moderate bulge at your side but again, most people will not notice.
Are you doing anything wrong? are you carrying in a place where you are not legally permitted to carry? If so, you have a problem.
If not, No Problem.
As a former MP, and as a member of the carry community I might notice your carrying, and I'm glad you are.
No worries. Carry what you shoot best and find a holster that carry's it comfortably.
If you have a permit its not against the law if your weapon is slightly protruding from your hip.
In the old days someone could claim you were "Brandishing" your weapon. Laws have changed in recent years to allow a CCW holder to carry without having to make the weapon 100% invisible.