A table runner that is too long ends up pooled in laps or dragging into the gravy, and one that is too short just looks like it gave up halfway down the table. That one measuring mistake puts more people off runners than anything else, and it is a shame, because the right one does something a tablecloth cannot, which is let the wood show.
The best thing a runner sits with is a table you actually like looking at, wood grain, an old farmhouse top, even a plain dining set that could use a little life. Lay it down the center of an everyday table, stoneware on either side, maybe a low bowl of fruit or a couple of candles in the middle, and suddenly the table has a shape to it instead of being one flat surface. It works just as well on a console in a hallway, under a tray and a lamp, or across a sideboard where it stops the whole thing looking like a shelf of leftover decorations. Runners are forgiving about theme too, a woven cotton one sits fine next to mismatched chairs, and a linen one doesn't mind sharing space with everyday plates rather than the good china.
For length, the rule that actually works is to let it hang eight to twelve inches past each end of the table, enough to read as intentional rather than left behind. Measure the table, not the chairs, and account for whether the table has leaves in or out, since a runner bought for a six seater will look stranded on a table stretched out for company. Width matters more than people expect too, anything under twelve inches reads thin and a little mean on a standard dining table, so look for fourteen to sixteen inches unless the table itself is narrow. The common mistake is buying for the table's length alone and forgetting the overhang, or centering it so carefully that it ends up looking stiff rather than lived in, a slight careless lay is almost always better.
Where a runner does not work is on a table that gets heavy daily use for actual meals with kids, plates sliding on and off a runner all day gets tiresome fast, and it is also the wrong move if the table itself is nothing special and needs covering rather than framing. For those tables, a full cloth is the honest answer.
The one mistake to avoid is guessing the length instead of measuring the table with the leaves in, because that is the runner you will end up folding under itself for the next five years.