The slider is defined by scale and method before it is defined by size. It is a small burger, two to three inches across, but the smallness is a consequence of how it is cooked: a thin patty steam-griddled over a bed of shaved onions in quantity, many at a time on one flat-top, the onions and meat cooking together under cover until the patty is done through and saturated with onion. This is not a shrunken hamburger. It is a different cook entirely, built for volume, and the patty's job is to absorb the onion and steam rather than to develop the hard dry sear a larger griddle burger is after.
The craft is in the steam and the onion. A pile of thinly shaved onion goes down on the griddle, the thin patty is pressed onto it, and the whole thing cooks under a lid or a tight pan so the rising onion steam drives moisture and sweetness up through the meat instead of searing it off. The patty is deliberately thin so it cooks fast and takes on the onion completely; a thick patty would defeat the entire mechanism. The bun is small, soft, and steamed near the meat so it goes pillowy rather than crisp, because the sandwich's whole character is soft, sweet, and onion-soaked, not crusted and beefy. Made in a tight row on one surface, dressed simply, it is engineered to be produced by the dozen and eaten by the handful, which is why the format exists at the size it does.
The variations stay inside the small, steam-onion-griddled idea and mostly change the topping or the protein. A pickle and a single small cheese slice are standard; some builds run a slider-sized chicken or fish cake, or a sweet party-roll version baked in a tray. It is one regional dialect of the American burger, distinct from the thin-pressed maximal-crust technique and from the cheese-stuffed and steamed-cheeseburger builds it is often confused with. Those deserve their own articles rather than being crowded in here.