Is 5 inches of girth big?
Yes. A 5-inch (12.7 cm) erect girth is bigger than average and sits around the 83rd percentile, meaning it's thicker than roughly 4 out of 5 men. The average erect girth is 4.59 inches (11.66 cm).
Yes. A 5-inch (12.7 cm) erect girth is clearly above average — about the 83rd percentile, thicker than four men in five.
The average erect girth is 4.59 in (11.66 cm), so 5 inches clears the mean by nearly half an inch. That figure comes from the largest clinician-measured dataset on record (Veale et al., 2015, BJU International, n=15,521), and at the 83rd percentile a 5-inch circumference sits firmly in “thick.”
One catch before you trust the number: girth is the distance around the thickest part of the shaft, not the width across it. Measure with a flexible tape (or a string you then lay against a ruler), and pull it snug rather than tight. People routinely overstate girth by squeezing the tape, so if 5 inches survives an honest measurement, you really are at the upper end.
For how circumference and length play off each other, see girth vs. length.