Size effect in carbon fiber/epoxy resin unidirectional composites was studied, by analyzing this effect in carbon fibers, as well as, in unidirectional composites themselves. Carbon fibers were tested by the single-filament tensile test with different gauge lengths (measured strengths and failure strains), while the unidirectional composite coupons of different dimensions were tested by standard static tensile tests (measured strength values). The obtained tensile strengths were analyzed according to the Weibull statistical strength theory for brittle materials, by comparing the variations in sample strengths (failure strains) during increase in samples dimensions with the variability scatter of results for individual samples around the mean values. Performed analysis has shown that the size effect in carbon fibers can fully be described by the Weibull theory. However, due to complex failure propagation, composites exhibit pronounced size effect, but followed by reduced strength variability. It makes detected composites size effect behavior different from that in brittle materials.