The percolating conductance of a new class of nanocomposite thin-film transistors, with channels composed of isotropic ensembles of nanotubes or nanowires, is analyzed as a function of wire/tube density and channel length. The conductance exponents are validated against analytical results for short channel transistors, and against available experimental data for longer channel devices. Our plots of conductance exponents as a function of tube-to-tube coupling strength provide a unified framework to interpret future experiments and should help design better nanocomposite transistors.