The size dependent colloidal behavior of aqueous dispersions of carboxylated multiwall carbon nanotubes (c-MWCNTs) is presented. The presence of carboxylic groups provided electrostatic stabilization in water, where the size affected agglomeration. While aspect ratio did not show any definite correlation, the hydrophobicity indices (HI), zeta potential and aggregation kinetics showed dependence on the length of the c-MWCNTs where the shorter c-MWCNTs showed significantly lower HI values, smaller particle aggregates, higher zeta potential values and higher critical coagulation concentrations (ccc) in the presence of electrolytes. Although the diameter of the short c-MWCNTs did not appear to influence their aggregation behavior, the longer c-MWCNTs showed a dependence on diameter where stability decreased with increasing CNT diameter.