Outer coatings that were as near alike as possible were applied to two different sizes of inert TRISO particles that were larger than those commonly used to fuel HTGR reactors and these particles were then irradiated in a test reactor to observe the influence of particle size on outer coating failures that resulted from irradiationinduced shrinkage of coatings onto the more stable Sic substrates over which they were applied. Outer coatings of plain pyrocarbon and of Si-alloyed pyrocarbon were used to make up two test pairs of particles with diameters of about 1050 and 1300 pm. For a fast-neutron tluence of 5.5 x 102-' n/m2 (E > 29 fJ) at an irradiation temperature of 1125K, failure was about twice as high in the larger 1300 pm particle of each test pair as in the smaller 1050 pm particle (16 vs 8%), with each of the coating types having roughly the same behavior. This observed size effect is somewhat greater than predicted by volume-dependent Weibull theory, which estimates failure of the larger size particles at 13% when the smaller size particles fail at a rate of 8%. However, experimental uncertainties are sufficient to account for the difference between observed size effects and those predicted by Weibull theory. The conclusion that the irradiation performance of coated particles is size dependent is reinforced by the fact that failure for regular 850 pm fueled particles in the same irradiation test was essentially zero.