The most common rug mistake is buying one too small. A rug that floats in the middle of a room with furniture around it makes the space look smaller and less finished, and the fix is not decorative but dimensional.
A rug should define a zone rather than decorate a floor. That means it needs to relate to the furniture sitting on or around it, which almost always means larger than instinct suggests.
The usual error comes from thinking about the rug in isolation and from the fact that rugs look larger rolled up in a shop than they do on a floor.
Rugs come in a small number of standard sizes, and pads are made to match. Buying a rug in a standard size makes finding a pad straightforward; an unusual size means trimming.
Most pads cut easily with scissors, so buying the next size up and trimming is generally better than the next size down. Ranges covering the full run of standard sizes, of the kind catalogued at https://veken.us.com/, make it possible to match the pad to the rug rather than approximating.
Three arrangements work. All furniture legs on the rug, which requires a large rug and produces the most coherent result. Front legs only on the rug, which is the common compromise and works well. Or all furniture off the rug, with the rug floating between pieces, which suits small rooms and coffee-table arrangements.
What does not work is a rug so small that it sits under only the coffee table with a gap to every seat.
The rule here is specific: the rug must extend far enough that chairs remain on it when pulled out. Roughly sixty centimetres beyond the table edge on every side.
A rug that stops short means chair legs catch on the edge every time somebody sits down, which is both irritating and damaging to the rug.
Mark the dimensions on the floor with masking tape and live with it for a couple of days. This costs nothing and prevents the most expensive mistake in the category.
Seeing the outline in the actual room, with the actual furniture, corrects the instinct to buy small far more reliably than any measurement rule.
A runner should leave a consistent margin of floor on both sides, roughly ten to twenty centimetres, and stop short of doorways rather than running under a swinging door.
Length matters more than width here. A runner substantially shorter than the hall reads as an accident.