Although size of tumor is a factor, the use of radiotherapy for treatment depends on the type of lung cancer. For non-small cell lung cancer, surgery, radiation, and/or chemotherapy may be used. Small cell lung cancer is more aggressive and radiotherapy and/or chemo are the choice of treatment.
For some patients, if surgery is not an option because of high risk of complications, radiation is the only option. So it depends on the type of tumor, its location, as well as the health condition of the patients. Best to talk to an oncologist about this.