I think they come up pretty normal verging on the generous, as my feet are wide and I take my usual size in them. However, given all the varying responses, maybe they come up differently in different models. Mine are the zip front Heritage jodhpur boots.
I ended up with a size 7.5 somehow! Im usually a size 6 in boots (wellies etc.) and a 7 in neater shoes. I think because they are quite narrow at the toe and I have wide feet.
they come up a tad small as Ariat have just changed to european sizing. I'm a size 5.5 (39) and the 39s fit perfectly in the foot (have wide feet) but are tight around the ankle when pulling them on (long boots). If you are a 6 I would definitely go for the next size up (40)
I have incredibly wide feet, and am normally a 7/8 because of this, although the actual length of my feet is a 6 1/2 both pairs of my ariats are this size...
Agree on the tight around the ankles, but they do give great support and give after a while...
I am sure you already know this but just in case you don't American sizing is different . My Ariats are a UK size 7 but on the box it also says US size 9 1/2.