The size effects due to changes in gauge length and the influence of the fragmentation phenomenon in fibrous structures are examined. First, a theoretical analysis of the differences of the size effects in single fibre and in a fibrous structure is conducted. Then comprehensive experimental work is presented on single fibres, fibre bundles, and twisted yarns which can be considered as pseudo-composites. Next a comparison is made between the theoretical predictions and the experimental data. Causes for the size effect in a fibrous structure are explored.