Why did the thieves especially want Oliver to help with the burglary?

In chapter 19, Bill Sikes explains that he wants a boy to help with the burglary of "that crib at Chertsey," and he says that the boy "mustn't be a big 'un." The implication is that Bill Sikes needs a boy small enough to fit through a panel on one of the doors or shutters to the house. Nancy then suggests that Oliver might be such a boy, also saying that "he's a safe one," meaning that he can be trusted.
Fagin agrees and pushes for Bill to take Oliver, in part so that Oliver can start "work[ing] for his bread." Fagin also says that all of the other boys are "too big" and that Oliver can be frightened into obedience. Bill agrees that Oliver is "just the size [he wants]" and assures Fagin that he shall indeed scare Oliver and kill him if he proves untrustworthy.
Fagin also thinks that it's a good idea for Oliver to be involved with the burglary so that he can start to "feel that he is one of us." Fagin says that, once Oliver has committed one crime, he shall be "Ours for his life," acknowledging that "it's quite enough for my power over him that he was in a robbery."
Fagin also says that Oliver is a useful boy to have in his gang because, with all the other boys, "Their looks convict 'em when they get into trouble." The implication is that Oliver is a good boy to have in the gang because he looks innocent and will, therefore, avoid detection or suspicion better than most.

Bill Sikes is initially somewhat reluctant to take Oliver along with him on his attempted burglary of the Maylie residence. The boy lacks experience in the ways of a criminal; and his lack of experience could easily lead to the gang's being rumbled by the police. But as Sikes has been casing the Maylies' place for two whole weeks without much success, he figures that it might be best to take Oliver along with him after all. Sikes reckons that the best way to get into the property will be through a small panel outside the house that can be lifted off, creating a little crawl space. Oliver is just the right size to crawl through that panel, enter the property, and then open the front door to let in Sikes and the rest of his gang of thieves.