We present a thorough experimental investigation of the phenomenon of soliton resonance sideband generation in femtosecond fiber lasers. The dependence of the sideband wavelengths on the dispersion and pulse length is confirmed. Third order dispersion is found to play a significant role in determining the sideband spectrum. We show that the minimum pulse length obtained in a fiber laser is determined by the cavity dispersion and relate this to loss into the sidebands. We show how the sideband spectrum can be used as a diagnostic of the fiber parameters and of the formation of ultrashort pulses in the laser.