There is growing evidence for the notion that large grains are an important constituent of the interstellar medium. It has been found to be necessary to invoke the presence of large grains (of radius a > 1.0 μm) in a number of circumstellar shells, including those around Eta Carinae, and some late type giants and supergiants. Most recently, as a result of IRAS observations, the same idea has been used in the interpretation of the extensive optically thin disks found around some young early type stars. It is important to establish size constraints on these grains, so as to determine better the probable mix of interstellar grains. Previous work on AG Carinae suggested merely that the grains in the circumstellar shell need to be larger than 1 μm, but gave no information regarding the maximum size which would be acceptable. In this paper, we explore the question of the characteristic size of the dust grains in the AG Carinae shell, by fitting the spatially integrated model energy distribution to observations over the wavelength range 360 nm–100 μm, and conclude that silicate grains of radius ~ 1.0 μm give a satisfactory fit to the spectrum and angular distribution, but that grains appreciably larger or smaller than this radius are unsatisfactory.