Detailed information on in-harbour shipping contribution to size segregated particles in coastal cities are scarce. In this work particle number and mass size distributions (size range 0.01-31 µm) were measured at high temporal resolution (1 minute) in two coastal cities of northern Adriatic Sea: Venice (Italy) and Rijeka (Croatia). Size distributions allowed focus in three size ranges: nanoparticles (diameter D<0.25 µm) ; fine particles (0.25<D<1 µm), and coarse particles (D>1 µm). Absolute concentrations were larger in Venice for all size ranges. Daily trends of concentrations showed a large influence of local meteorology and boundary-layer dynamics in Venice and a contribution of road transport larger (in relative terms) in Rijeka. The contributions of shipping were significantly larger in Venice mainly because of the larger ship traffic. Maximum impact was on nanoparticles 7.4% (Venice) and 1.8% (Rijeka), the minimum was on fine range 1.9% (Venice) and <0.2% (Rijeka) and intermediate values were found in the coarse fraction 1.8% (Venice) and 0.5% (Rijeka). Contribution of shipping to mass concentration was not distinguishable from uncertainty in Rijeka (< 0.2% for PM1, PM2.5, and PM10) and was about 2% in Venice. Relative contributions as function of particles size show remarkable similitudes: a maximum for nanoparticles, a quick decrease and a successive secondary maximum (2-3 times lower than the first) in the fine range. For larger diameters, the relative contributions reach a minimum at 1-1.5 µm and there is a successive growth in the coarse range Size distribution showed a non negligible contribution of harbour emissions nanoparticle and fine particle number concentrations, compared to PM2.5 and PM10, indicating them as a better metric to monitor shipping impacts compared to mass concentrations (PM2.5 or PM10)