-Shrug-
Being a novice preset maker, a question like this has never really crossed my mind. I can see how increasing the file size could increase the quality of the preset design. If it were to be increased, the preset won't look so pixelated and blocky when it comes to the marking designs and tiny details. But of course, taking up more MBs would take up more memory storage on a computer.
I also think that saving the preset image as certain files contributes to the pixelations of the designs. I've noticed that if I save them as jpeg. images, the markings (especially if they're red) will become pixelated. If I save them as a gif. or a bmp. files, it doesn't really happen as much.
I don't know exactly how many MB it takes to be a "good" preset file. Probably something somewhere between 4-6 MB. It doesn't sound like something too big or too small, or too much to download... but everyone's computers act differently when it comes to downloading. I'm not that much of a computer specialist, I just know how to operate one. And if it stalls during downloading, I smack at it to get it to go faster.