The influence of process conditions and reactor scale on the performance of stagnation flow Chemical Vapour Deposition reactors has been studied with the use of a numerical model for the transport phenomena and chemical reactions. Special attention is given to the formation of reaction intermediates and byproducts, both in a generic CVD process, and in two specific processes for the deposition of silicon and tungsten films. Scaling rules are presented for the thermal and concentration boundary layer thickness, and for the concentration and uniformity of reactant, intermediates and byproducts. It is concluded that scale-up of the diameter of stagnation Row CVD reactors should be performed at fixed Peclet number, and at fixed reactor height rather than fixed diameter-to-height ratio. In this case the gas flow rate should be increased proportional to the square of the diameter.