The beam quality of an idealized two-dimensional field emitter is evaluated. Under the assumption that the space-charge effects are negligible, a scaling law is constructed for the intrinsic emittance of an isolated emitter. This scaling law takes into account the Fowler-Nordheim (F-N) emission law and the effects of local field enhancement. It is given in terms of the height and width of the emitter, anode-cathode separation and voltage drop, as well as the F-N coefficients A and B.