Rugs and scale
The most common mistake is buying a rug that's too small. A small rug makes a small room feel smaller still.
The furniture has to sit on it
A rug that floats in the middle of a seating arrangement, touching nothing, reads as a mat — it shrinks the room visually rather than anchoring it. The front legs of every sofa and chair should sit on the rug. That single rule forces you toward a size that actually works, and it usually means going one size larger than your instinct suggests.
In a dining room, the rug needs to be large enough that a pulled-out chair still has all four legs on it. Most people underestimate this by a clear thirty centimetres on each side. Measure the table, add the depth of a chair pulled back to sitting position, and then add a margin — that is your minimum.
In a bedroom, the usual placing is two-thirds under the bed, protruding enough on both sides and at the foot that bare feet land on it when getting up. A rug that only peeks out from the sides of the bed is not doing any work.
How to test it before you buy
Tape out the footprint on the floor with masking tape before committing to anything. It takes five minutes and it is consistently surprising — the size that looks right in tape almost always feels larger than what you would have ordered without it. Pull the furniture into position over the tape and stand in the doorway. That is the view that matters: a rug that is correctly sized will read as part of the room's structure rather than an object sitting in it.
Tape out the footprint on the floor with masking tape before committing to anything.
Colour and pattern are details. Getting the footprint right is the whole job.