The thermally induced optical transmission changes upon melting and solidification of Bi nanostructures embedded in amorphous Al(2)O(3) thin films have been studied as a function of characteristic sizes in the range 7-35 nm. The results show a repeatable optical transmission-temperature hysteresis loop with sharp changes at the melting and solidification temperatures. Both temperatures decrease linearly when the mean size of the nanostructures decreases and they are respectively higher and lower than that of the bulk melting temperature of Bi. The optical transmission shows a significant contrast that increases up to 16% as the mean size of the nanostructures increases. The results show that in addition to the usual decrease of melting temperature as the size of the nanostructures decreases, the melting temperatures for all samples are higher than that of the bulk. This unexpected result is associated to the contraction of Bi upon melting and to matrix effects related to the balance of surface energies between the solid and liquid Bi and the matrix. The solidification process shows a weaker dependence on the nanostructure size, and thus it is consistent with a mainly volume controlled process. It is shown that by controlling the characteristic dimensions of the Bi nanostructures the temperature operability of the Bi nanostructures embedded in amorphous Al(2)O(3) as a thermally driven optical switch can be tuned over 73 K.