We describe the optimization of transition edge superconducting (TES) detectors for use in a far-infrared (FIR) Fourier transform spectrometer (FTS) mounted on a cryogenically cooled space-borne telescope (e.g. SPICA). The required noise equivalent power (NEP) of the detectors is approximately 10 -19 W/√HZ in order to be lower than the photon noise from astrophysical sources in octave wide bands in the FIR. The detector time constants must be less than 10 ms in order to allow fast scanning of the FTS mechanism. The detectors consist of superconducting thermometers suspended on thin legs of thermally isolating silicon nitride and operate at a temperature of approximately 100 mK. We present the design of the detectors, a proposed focal plane layout and optical coupling scheme and measurements of thermal conductance and time constant for low NEP prototype TES bolometers.