Polymeric nanoparticles (NPs) are highly engineered nano emulsions with applications in several fields. The control over NP surface chemistry, size (Dn), and molecular weight (MW) of the polymer they are made up of plays a paramount role in the optimization of their end-use performance. In this work, the theoretical basis to decouple the dependence between the NP Dn and MW has been presented, and an operative way has been demonstrated via ab initio reversible addition-fragmentation chain transfer (RAFT) emulsion polymerization (AIREP), a "living" heterogeneous process that adopts RAFT macro-surfmers: macromolecular chain transfer agents (CTA) produced via RAFT polymerization of amphiliphic monomers, such as surfmers. The possibility of obtaining the desired length of the lipophilic block or the length of the whole block copolymer and the NP Dn by choosing the correct length of the RAFT macro-surfmer has been assessed. It has been discovered that a wide range of Dn and MW can be achieved, but not very big NPs with very low copolymer MW; this limit is consistent with the physical and geometric constraints of the system.