A 2" hole right in the middle would work, with just a small pipe of length about 2" as well. More is not needed because it'll be 1 pipe right at the top.
However I would opt to not do that as the heat will shoot up and you'll burn a little more fuel.
My friend and I both have built ones where we have 2 holes on the side of the barrel, each of those holes big enough to thread 1.5" iron pipes. Then do an elbow to go up with another short nipple on it so that you can use the cap (instead of the plug) to close it. These holes on the side would be 2-3 inches down from the top, and I'm referring to the top of the hole. So that the smoke has to come out by going down a little before going out.
This has worked great at keeping things moist inside and evening the temp and being fuel efficient. Plus, if you make the holes exactly opposite side of each other, you could remove the elbows and insert a rotisserie rod through it. Don't necessarily need to use the elbows.
If it's outside, you should either spray it with oil or paint it or use galvanized. The temps will not get anywhere close to hot enough there to have health issues with the galvanized. Painting would be for the outside. The point is to prevent the iron from rusting.
Shopping list for the side exhausts:
1.5 inch iron nipples x 4
1.5 inch iron 90 elbow x 2
1.5 inch iron cap x 2
To drill hole to fit just right, you typically need about 1/4" to allow for the iron piping, so you'd have to drill like 1.75" hole.