From the Guidelines

A 3 cm lung nodule on CT scan is defined as a well or poorly defined rounded opacity measuring exactly 3 cm in diameter.

According to the American College of Radiology 1, a pulmonary nodule is defined as a well or poorly defined rounded opacity measuring less than 3 cm in diameter.
The British Thoracic Society guidelines 1 also define a pulmonary nodule as a well or poorly circumscribed, approximately rounded structure that appears on imaging as a focal opacity and is less than or equal to 3 cm in diameter.
The American College of Chest Physicians 1 defines a solitary pulmonary nodule as a single, well-circumscribed, radiographic opacity that measures up to 3 cm in diameter.

Lung nodules can be characterized as solid, part-solid, or ground-glass on CT, based on their attenuation, allowing for a more accurate assessment of malignancy risk 1.
The size and morphology of the lesions, as well as risk factors for malignancy and suitability for curative treatment, are important considerations when evaluating individuals with lung nodules 1.

CT is widely recognized as the modality of choice to evaluate pulmonary nodules, with thin-section CT being estimated to be 10 to 20 times more sensitive than standard radiography 1.
Low-dose technique is recommended for CTs performed to follow lung nodules, and IV contrast is not required to identify or initially characterize pulmonary nodules in clinical practice 1.

From the Research

A 3 cm lung nodule is considered a relatively large nodule, and its size is an important factor in determining the likelihood of malignancy 2.
According to a study published in BMC Cancer, solid lung cancerous nodules with a diameter of 2.0-3.0 cm have distinct CT features, including a regular shape, homogeneous density, and clear but coarse tumor-lung interface 3.
The probability of malignancy for a nodule of this size is not explicitly stated in the provided studies, but it is generally higher for larger nodules 2.
Management options for a solid pulmonary nodule of this size may include surveillance imaging, positron emission tomography-CT imaging, nonsurgical biopsy, and surgical resection, depending on the estimated probability of malignancy and patient preferences 2.

The CT features of lung nodules can vary depending on their size, with smaller nodules tend to have fewer suspicious features than larger nodules 3.
A study published in Radiology found that low-dose CT is acceptable for pulmonary nodule identification, but may have reduced diagnostic sensitivity for peripheral nodules and nodules separated from blood vessels 4.
Another study published in the Journal of Thoracic Imaging found that short-term interval CT follow-up is not necessary for small incidental pulmonary nodules (< or =4 mm) in patients with no history of malignancy or acute lung disease 5.

The size of a lung nodule is an important factor in determining the likelihood of malignancy, and larger nodules are more likely to be malignant 2.
The management of a lung nodule depends on its size, as well as other factors such as patient risk factors, imaging characteristics, and clinical judgment 2.
Further evaluation and follow-up are necessary to determine the nature of a lung nodule, especially for larger nodules or those with suspicious features 2, 3, 6.