usually, you will find target size varies with distance and time on barricades. MDs generally don't like to put steel closer than 200-300 yards, and preferably out a bit further, just because it's hard on the steel and target frames and you'll have more target malfunctions in a match with closer steel.
my guess is you'll see typically a 2-3 MOA target off most barricades, depending on how stable the position is and how much time they give you. i'd favor the small side of that when practicing, of course. 5 ports in a wobbly piece of plywood in 60 seconds? prob a 3 MOA target. 5 shots off the same rock or steel or railing that supports both front and rear of the rifle in 60 seconds? prob a 2 MOA flapper behind a hostage. but i have seen stages where you got a 1.5 MOA plate at 1000 yards too.
6" and 8" circles would be good to practice on at 300-400y
for practice, dots on paper are fine too. honestly, i do most of my barricade practice dryfiring at a spot on the chair rail in my dining room