We investigate the potential of using ratios of fine structure and near-infrared forbidden line transitions of atomic carbon to diagnose protoplanetary disk extension. Using results from 2D photoionisation and radiative transfer modeling of a realistic protoplanetary disk structure irradiated by X-rays from a T Tauri star, we obtain theoretical emission maps from which we construct radial distributions of the strongest emission lines produced in the disk. We show that ratios of fine structure to near-infrared forbidden line emission of atomic carbon are especially promising to constrain the minimum size of gaseous protoplanetary disks. While theoretically viable, the method presents a number of observational difficulties that are also discussed here.
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