We present an experimental study of the vortex-motion dissipation in a mesoscopic amorphous (a-)Nb0.7Ge0.3 strip, with emphasis on the results for 3-8 vortex rows parallel to the long strip axis. In the isothermal voltage vs magnetic field traces, at a constant current, we observe plateaus superimposed onto a monotonic background. The plateaus appear because finite vortex-core size influences the accommodation of the flux lattice into the strip. This conclusion is drawn from a quantitative analysis, which is free of adjustable parameters, of the magnetic fields that edge the plateaus.