Poly(ethylene glycol) (PEG) functionalization of carbon nanotubes (CNTs) is widely used to render CNTs suitable as vectors for targeted drug delivery. One recently described PEGylated version uses an oxidized single-walled carbon nanotube called a hydrophilic carbon cluster (HCC). The resulting geometric dimension of the hybrid PEG-CNT or PEG-HCC is an important factor determining its ability to permeate the cellular membrane and to maintain its blood circulation. Molecular dynamics (MD) simulations were performed to estimate the maximum length and width dimensions for a PEGylated single-walled carbon nanotube in water solution as a model for the PEG-HCC. We ensured maximum PEGylation by functionalizing each carbon atom in a CNT ring with an elongated PEG molecule, avoiding overlapping between PEGs attached to different CNT rings. We suggest that maximum PEGylation is important to achieve an optimal drug delivery platform. © 2012 American Chemical Society.