Abstract: Tropical cyclone (TC) has brought huge losses to coastal areas, whose intensity and size are both important indicators of destruction. The Northwest Pacific is the area with the most TCs generated. Due to the lack of effective observation methods and monitoring information, the TC operational centers of coastal countries or regions have not yet established complete TC outer-core size prediction and testing service. Thus, to select factors that significantly impact TC size changes and improve TC size forecast, statistical analysis is carried out on the climatological characteristics and the lifetime covariation characteristics of intensity and outer-core size (selected as the radius of damaging-force winds, R₂₆) over the Northwest Pacific from July to November during 2004-2020, using the tropical cyclone best track data from JTWC (Joint Typhoon Warning Center) and SHIPS (Statistical Hurricane Intensity Prediction Scheme) reanalysis data. The results show that TC intensity and size peak in October, mainly showing a higher proportion of strong and large-sized TCs with longer lifetime at sea than in other months. Generally, the TC size expands with the increase in intensity and shrinks as the TC weakens. TCs reach the lifetime maximum size (LMS) later than the lifetime maximum intensity (LMI), with a mean lag time of 40 hours. Compared to the TC rapid intensification and LMI, the mean meridional positions of TC rapid expansion and LMS are closer to the coastal continent. Initial vortex size of TC affects the size development, especially LMS. Specifically, 58% of small initial vortices maintain the size in the small to medium category, while 71% of vortices with large initial size develop to large vortices in later periods, with 59% intensify to strong TCs (no less than 59 m·s^-1) at LMI stage. Compared to small initial vortices, vortices with larger initial sizes tend to attain the greater integrated kinetic energy. The size of the latter stage has a high correlation (no less than 0.45) with the initial R₂₆ for 66 h, indicating that the initial size of TCs can be a key predictor. The peak of size change rate (ΔR₂₆) is located at moderate intensity (25~50 m·s^-1) and the peak intensity change rate (ΔV_max) is located at medium and small size (50-100 km). The outer-core size is more likely to expand outward and even leads to rapid expansion under the conditions of stronger upper air divergence, higher relative humidity, larger ocean heat content and moderate vertical shear.