Girth would be the circumference around your two widest points on the foot, or the two joints. However, measuring this it should be at an angle and not straight across the foot as the two joints are not perfectly across from each otherWell I'm 272mm long, 100mm wide, approx 152mm girth (this one is weird to measure so not sure). I can't fit into a 9UK HIRO without it destroying my mid-foot, while 9.5 is just long and awkward. RON in 9UK is also very narrow for me. Meanwhile I feel fine in 8.5UK in HOK. I can't imagine how someone with a foot even wider than mine can wear HIRO or RON.
Typically I size as a 10D or 9.5E US, so that would be my size for most lasts from Allen Edmonds. I'm also 9.5 in Viberg, 9.5D in Alden Barrie, and 9.5D/9E in various pairs from Grant Stone.