The influence of the polymer length and the valency of guest-modified poly(ethylene glycol) (PEG) on the stability, size tunability and formation dynamics of supramolecular nanoparticles (SNPs) has been studied. SNPs were formed by molecular recognition between multi- and monovalent supramolecular building blocks with host or guest moieties, providing ternary complexes of cucurbit[8]uril, methyl viologen and naphthol (Np). SNP assembly was carried out using monovalent Np-modified oligo(ethylene glycol)s and PEGs with 3 or, on average, 18, 111, or 464 ethylene glycol (EG) repeat units. SNP formation and stoichiometry-controlled size tuning were observed for SNPs prepared with Np-modified PEGs containing between 18 and 464 EG repeat units, whereas no distinct assemblies were formed using the shorter Np-functionalized tri(ethylene glycol). Tentatively, the stabilization of SNPs by monovalent PEGs is partly attributed to dynamic exchange. Use of the divalent Np-functionalized PEG (with 113 EG repeat units) slowed down the SNP assembly dynamics and distinct sizes were only obtained when performing the self-assembly at 40 °C for 12 h.