Sugar-responsive polymer fluorescent nanospheres based on N-isopropylacrylamide and phenylboronic acid were prepared by precipitation polymerization with an average size of 380 nm. The particles swell in the presence of sugar, and the rate and extent of this swelling was measured with distance-dependent fluorescence resonance energy transfer. We present a direct link between the ratio of the intensities of the fluorophores and the particle size. Depending on the kind of sugar and the sugar concentration, different swelling kinetics and equilibrium sizes are observed. With this approach we can obtain insight into complex structure changes with simple fluorescent measurements.