Understanding Targets

OK.  Here I go again with another newb question.
I have been looking at buying some practice targets.  While looking at NRA certified targets, I see that there seems to be 2 different sizes of 25yd targets.  Specifically B16 (small) and B8 (large).  Which one should I buy?  I have been shooting out behind my house.  I have plenty of room to set targets at both 25 and 50 yards.  
Please advise.
As always thanks
Doug

B16 is for slow fire and is normally used for indoor matches since most indoor ranges aren't 50 yards. The B8 is for timed and rapid, indoors and out. The B6 is for slow fire at 50 yards.

So, are you saying that B16 is to simulate a 50 yard target in a 25 yard indoor range?

http://www.indecorous.com/bullseye/rings.html

try here.

James
Thanks so much!  It makes much more sense to me now.
Doug