Monodispersed Yb(3+)(20%)/Er(3+)(2%)-doped BaLnF(5) (Ln = La, Gd, Lu) nanocrystals were synthesized by a thermal decomposition method. The study reveals that the size of the nanocrystals scales down with the Ln(3+) size without affecting the phase and shape of the nanocrystals. This is supported by the microscopic and optical properties.