Smallest netblock that providers will accept?

Hello All, I am curious as to the routing polices of the bigger providers such as ATT, L3, Internap, Qwest, Etc Etc... Is there a standard size netblock that these providers will accept? For instance, if customer "A" gets a /22 from ARIN and his upstream provider is ATT and L3, what would the smallest block be that those providers would accept from customer "A"? Would they accept something as small as a /24? Thanks, Mike