Poly(phenylacetylene) bearing second generation lysine dendrons through urea groups (poly-2) was demonstrated to be a superior anion receptor possessing strict size specificity in colorimetric anion detection. On the basis of the comparison of the colorimetric response behavior between poly-2 and poly-1, which is poly(phenylacetylene) bearing first generation lysine dendrons, the size of the pendant dendron was implied to dictate the selectivity in the colorimetric anion detection. Various instrumental analyses including H-1 NMR, light scattering, and viscosity measurements concluded that the conjugation of the bulky G2 dendrons in the vicinity of the urea receptors plays a crucial role in imparting the strict size specificity in the anion-sensing ability of poly-2.