A lot depends on your build and how you dress.
I carry a full size Glock or 1911. That said, I am 6'6" tall and weigh 300 pounds. I'm fairly certain I could conceal a Howitzer. I normally carry either IWB or OWB on my right side about 3 o'clock under jeans and a button up or polo shirt untucked with an undershirt tucked in to protect the weapon from sweat- and it's more comfortable IMHO.
A longer cut shirt helps out a lot, so look for tall sizes next time you are at the mall. In the summer it is almost always IWB for me. Cargo shorts with a (again) longer polo shirt do the trick for me.
Some other tips others have given me...
-Do not skimp on your holster and belt. A good stiff belt with a quality holster will be more comfortable and easier to wear than the lesser quality stuff.
-Avoid nylon. Leather or kydex are the way to go.
-Shirts with patterns/stripes will break up the outline of your weapon if it is printing.
-If you are carrying a big black gun avoid thin white shirts.
-Don't adjust your holster in public, even if it is under a shirt. Go to the mens room if you must.
-If the gun is too big to carry, get another gun. If it is not comfortable or you cannot conceal it properly, it will only cause trouble.
-Most important of all, Concealed means concealed
Hope I have helped.