Is a 6-inch penis big?
Above average: longer than 89.9% of men. By most people's standards this is a good size.
If it is the soft length
Measured soft, 6″ is longer than 99.9% of flaccid penises. Flaccid size says little about erect size, though: some grow a lot when hard, some barely.
Girth and condom size
Length does not decide condom size; girth does. At the average erect girth of 11.7 cm (4.6 in), a condom about 53 mm wide fits. Your own girth gives your own size. Condom size calculator →
How to measure it right
Measure erect, standing, with a rigid ruler along the top, pressed into the fat pad to the pubic bone, and read it at the tip. The calculator has the full guide, girth included. Penis size calculator →