With Snake Feint:
At 3rd level, a snakebite striker who uses a standard action to move can combine that move with a feint. If she is able to feint as a move action (such as from having the Improved Feint feat), she can combine a move action to move with her feint. At 11th level, once per round she can declare her square and one adjacent square as the origin of her attacks until her next turn (allowing her to use one or both squares to determine whether she or her allies are flanking an opponent). At 15th level, she counts an additional adjacent square for this purpose.
What if the Brawler is large (or bigger). Obviously, that means that 'her square' is still 10 ft wide. What about her other virtual squares? Are they also 10 ft wide, or are they 5 ft?
I would argue that a large character's "square" is actually its SPACE, and its space is four squares on the standard playing grid... so this ability would allow it to count a 5th adjacent square as the origin of its attacks at 11th level, and a 6th adjacent square at 15th level.
I do not see a large character with this ability commanding 12 squares at 15th level.
I guess the writer had only the iconic swashbuckler in mind - which is a Medium sized half(?)-elf. And the Advanced Class Guide had a bad start (as far as I heard), so even after years of fixing there might be some more minor oddities left.
Squares are intended to be 5-foot-sized:
There are three movement scales, as follows:
• Tactical, for combat, measured in feet (or 5-foot squares) per round.
• Local, for exploring an area, measured in feet per minute.
• Overland, for getting from place to place, measured in miles per hour or miles per day.
Considering the express use of the feat is for flanking (and feinting), and you're supposed to draw flank from the center of your square normally (squares if you're large or bigger), I would assume that you aren't getting to pick a single square of your space normally or and adjacent space with the feat, you're picking the center of your space or the center of an adjacent square (which would then be just a single square and work as normal for a medium creature).
That said, adjacency for large creatures probably should be based on their actual reach if not space, as it is extremely dumb that say a giant cannot cleave something that is 10ft away (ergo not adjacent) from the original target, the same relative distance away from its first target as two medium creatures are another for another cleaving medium creature, or gain the benefits of adjacency say for teamwork feats for any other creature it can naturally reach as if the grid were scaled up to its size.
But this is just a case of size and spacing being never properly addressed for creatures bigger than medium and smaller than... uh... small. Just a GM and table by table adjudication that needs to be added to everyone's session zero.
By the same "rule" you can't arbitrarily say medium creatures are the default. (Also I was clearly talking from houserule territory.) Two fine creatures can be 20 times their natural reach of another and still be "adjacent" but two colossal dragons are 10ft away from another and suddenly aren't adjacent because some puny humans said the world exists in 5ft quantum tunnels of spacing.
Again, fairly common houserule (in my experience) to get rid of the silliness of size and spacing.