This study aimed to identify the optimal diameter threshold for defining positive results in solid nodules at baseline screening low-dose CT (LDCT) and to compare the effectiveness of two-dimensional and volumetric measurements of lung nodules in diagnosing lung cancers. The study population consisted of consecutive participants from the Korean Lung Cancer Screening project between 2017 and 2018. Semi-automated segmentation was used to measure the average transverse diameter and effective diameter of lung nodules using the A-view Lungscreen software from Coreline Soft. The diagnostic performances for lung cancers, diagnosed within 1 year after LDCT, were evaluated using area under receiver-operating characteristic curves (AUCs), sensitivities, and specificities. The study found that elevating the diameter threshold for solid nodules from 6 to 9 mm could significantly increase the specificity and modestly reduce sensitivity, resulting in a 60.2% estimated reduction of unnecessary follow-up LDCTs, and a diagnostic delay in 1.9% of lung cancers. Thus, the results suggest that increasing the diameter threshold for solid nodules may lead to a substantial decrease in unnecessary follow-up LDCTs with a small proportion of diagnostic delay of lung cancers.