We present a one-dimensional hydrodynamic model of the intense laser-cluster interaction in which the laser field is treated self-consistently. We find that for clusters initially as small as approximately 25 A in radius nonuniform expansion of the heated material results in long-time resonance of the laser field at the critical density plasma layer. A significant result of this is that the ponderomotive force, which is enhanced at the critical density surface, can be large enough to strongly modify the plasma hydrodynamics, even at laser intensities as low as 10(15) W/cm(2) for 800 nm laser pulses. Simulations of our recent experiments in extreme ultraviolet and x-ray generation from clusters explain the dependence of generation efficiency on laser pulse width.