We present a theoretical model to describe the feature size produced by direct laser writing upon a photoresist relative to various experimental parameters. The model allows the number of parameters required for describing the linewidth to be reduced and shows how the description can be made in terms of the ratio of laser power to writing velocity. Both of the limiting cases of the truncation of the laser beam are analyzed; i.e., the case of a nontruncated (Gaussian) beam and the case of a strongly truncated beam (simplified with uniform illumination assumed). Experimental measurements are presented that are fitted to the model to permit its validity to be assessed and for a comparison of these two regimes, which are shown to be different.