I only have 25, 50 or 100 yards to shoot with my AR. I am getting used to it and the iron sights, and now want to try and do more than just hit the paper. What target/bull sizes should be used at these distances? Is the 100 yard black size twice as big as the 50 yard black size? Can I use a 1/2 size target at 100 yards to duplicate 200 yards? I just don't know much about proper AR targets other than trying to hit the paper. Thanks.

Use the M16A2 zero target (to zero obviously)
and the 25m M16 qualification target.  The furthest silhouette is scaled at 300m.

Both are shot at from a distance of 25 meters.

I got mine from [url=http://www.letargets.com/html/targets1.html]Law Enforcement Targets[/url]

Both are listed at the bottom right of the web page.

how big are your groups?
think in MOA or minute of angle
basicaly it's
1" at 100yds
2" at 200
6" at 600
10" at 1000

so if you shoot 2" at 100 yds you can expect to his a 8" target at 800 yds.  it's just that easy.  what size target do you want to hit and at what range.  1MOA is a magic number for some reason.  when it it used "under field conditions"  it defines a target or sniper rifle.  lot's of guns shoot SUB MOA off the bench but put them in the mud and they go to hell.  a sniper rifle needs to shoot tight in the field.  

next weeks lesson, doping the wind without looking like a dope.  

and for 50 you just double you MOA
so a 2" group at 50 is a 4" group at 100