Ct scan showed that I have a 6cm spot/growth on the lower lobe of my right lung. Healthy 40yr old non-smoke fem. Is that large? How common is this?

6cm=big; 6mm=small. Sometimes people have a 6 mm spot (about 1/4 of an inch wide) but mistakenly remember it as a 6 CM mass (about 2-1/4 inches wide). A mass 6 centimeters (cm) wide is pretty big (a tennis ball is about 6-1/2 cm) and is usually not called a "spot". The doctor who ordered the ct can review the results to confirm whether the finding is a little 6 millimeter (mm) spot or a bigger mass.
About 3 inches. That is a pretty big area...About the size of a tennis ball. Although possibly nothing, it certainly deserves a work-up to know for sure. I'm sure your doctor will want a biopsy of this to identify exactly what it is and determine what/if treatment is necessary.