Durum wheat quality, especially milling performance, is largely affected by kernel physical properties. Due to overall dry growing conditions in 2017 and 2018, the majority of durum wheat produced in the Canadian prairies was high in protein content and met the test weight (TW) requirements for the top grades of Canada Western Amber Durum (CWAD). However, some durum samples, although graded as No. 1 or 2, had smaller kernel size (KS) and were significantly lower in milling yield and semolina quality. This study was conducted to investigate the impact of kernel physical characteristics, with a focus on KS and its distribution, on durum milling potential, semolina and pasta processing quality. Three sets of durum samples were used. The first set was a blend of commercial cargo loading samples. To investigate the kernel size distribution (KSD) in relation to functionality, kernels were segregated based on their sizes by sieving. The second set was composed of blends from two commercial durum wheat samples with low and high TW. The third set included composites of four CWAD varieties (Transcend, Strongfield, Brigade and CDC Verona) segregated based on their TW (75 to 84 kg/hL) and protein content (10.3 to 18.7%). Results showed that KSD is a better predictor for milling quality than TW, although they are strongly related. The kernels passing through a 2.38 mm sieve had much lower semolina yield than those retained, and the resulting semolina had significantly higher ash content and dull colour. Protein content is strongly related to the proportion of kernels passing through a 2.38 mm sieve. In addition, a drastic increase in the amount of kernels smaller than 1.98 mm was noticed when TW decreased from 79 to 76 kg/hL. These small kernels had a very detrimental impact on overall quality. Spaghetti prepared from samples with a high proportion of small kernels was firmer, but less bright, more brownish and less yellow mainly due to the elevated ash content and higher protein level.