I mean the only issue I've ever had with heads was one ex who would balloon out when I was deepthroating him. That was a tad challenging but I got used to it. Girth can certainly be more challenging but only in terms of the distance between the back molars. Obviously the smaller the person's mouth is generally, the smaller that gap will be and no one is gonna want a toothy bj. Once you're past that teeth gap, ,the throat can stretch quite a bit. Shape can be an issue but more in terms of what sort of curve it is.
Speaking from personal experience, as I am very large. I've only been deepthroated a handful of times, but there are some really impressive sword swallowers out there.
Some of the guys on here though seem to use it as a weird humble brag. I remember seeing one guy boasting about getting half the head in a woman's mouth and claimed it was "good head". Nope, that is shit head and it's weird to brag about getting bad head.