She is the one who controls who comes and goes through the river by altering its size when she is paid. Without payment, the river is infinite. Komachi was not present for this river crossing at all, therefore it's infinite in size.
"The speed of your boat is also slower than the rest of the shinigami." (PoFV, Eiki's Scenario, vs Komachi)
"The more you pay the ferryman, the shorter the distance. Plotting the curve of this inversely proportional function, we see that the closer to zero the ferryman receives, the closer the distance approaches infinity. Conversely, the more the ferryman is paid, the closer the distance approaches zero." (from the Bohemian Archive in Japanese Red article)
Komachi is one of many Shinigami and one of many ferrymen, all of whom are implied to have the same ability to control its width by quotes such as these. Any of them could've manipulated its width, and that's assuming it had to be manipulated at all. This also does not necessarily mean that the river's width is inherently infinite, just that the Shinigami will make its width bigger and bigger if they're not paid properly.
Also,
"This river has no fixed width. Rather, it is famous for extending to extreme distances under certain circumstances. Because of this, calculating the width of the river was assumed to be impossible." (Bohemian Archive)
This implies the river's width is not inherently infinite, but can merely increase infinitely.
But, you know, again, this also mentions that the river has no fixed width.
Discussion rule or not, I will continue to argue against things that are blatantly wrong and misrepresenting facts.