Is a 6mm pulmonary nodule in the right upper lobe too small to biopsy?

Probably so: Biopsy of a lung nodule depends on the size and location. A nodule smaller than 1 cm is difficult to biopsy. A nodule located too close to the heart or main blood vessels is difficult to biopsy. A nodule too adjacent to the ribs may also be difficult to biopsy. In your case ct follow up at intervals by established guidelines may be considered.