if all that you have is durablocks then you get the 2-3/4 rolls, which are called sheet rolls, the other discs are for dual action sanders, so i am not sure why you do not know which one you need.
check out
www.autobodytoolmart.com
http://www.refinishmaterials.com/ct_catalog.htm
the 2nd site has the carbo for what i pay for it as a body shop, you can get cheaper paper but you will go thru alot more of it