Almost every difficult dining room is difficult for the same reason. The table was chosen for the number of people who come at the busiest time of the year, and it then sits at that size for the other fifty weeks, leaving no room to get behind a chair. The room is not too small. The table is doing the wrong job.
Start with the clearances, because they are fixed by bodies rather than by taste. A person needs about sixty centimetres of table edge to eat without knocking elbows. Pulling a chair back and sitting down needs about seventy-five centimetres behind the chair. Walking past someone who is already seated needs about ninety. If your room cannot give you ninety on every side, decide which side is the walking side and protect it, then let the other sides be tighter. A table pushed toward a wall with seating on three sides is a completely respectable arrangement and is much better than a centred table nobody can get around.
This is the argument for a table with leaves, and it is a strong one. An extending table lets you live at the size the room wants and grow to the size the year occasionally demands. Older extending tables are common secondhand because they went out of fashion for a while, which means the mechanism is often better made than the price suggests. Open and close it in the shop; a leaf that binds when cold will bind every time.
Shape matters more in a small room than in a large one. A round table of the same seating capacity takes less floor area than a rectangular one and removes the corners that people walk into, but it is harder to push against a wall. A rectangular table seats more people per square metre and sits neatly under a window. An oval splits the difference and is the most forgiving shape in a narrow room. Draw the footprint on the floor in masking tape before you buy anything. It takes ten minutes and it is the only reliable way to feel a size.
Chairs are where small rooms are usually rescued. A matched set of six substantial chairs will fill a small room whether or not anyone is sitting on them, because they occupy floor space permanently. Two armchairs at the ends and a bench along one side carries the same number of people and disappears when not in use, since the bench slides under. Mixing chairs is not a compromise here, it is the better answer: different heights and materials read as a room that accumulated rather than a room that was ordered.
Watch the back height. Tall chair backs are the single most effective way to make a small dining area feel enclosed, which is sometimes what you want and is usually not. In an open-plan space, low backs keep the sightlines open across the room and make the dining area read as part of the whole. In a separate small room, tall backs can be used deliberately to make the space feel like a proper room rather than a leftover corner.
Lighting is the last piece and the one most often left at whatever the previous occupant fitted. A pendant hung roughly seventy-five to eighty-five centimetres above the table puts light on the food and faces without putting a bulb in anyone's eye. Hung higher, it lights the room and stops defining the table. Hung lower, it divides the people sitting across from each other. If the table extends, remember that the pendant will no longer be centred on the extended top, which is an argument for a linear fitting or for two smaller pendants rather than one large one.
The overall aim is that the room should be comfortable at its everyday size and capable at its occasional size, rather than permanently arranged for a night that happens twice a year. Once you accept that, a small dining room stops being a problem to solve and becomes one of the easier rooms in the house.