There are no exact rules, but here’s a guide: The average person needs 18 inches from the table edge to the front back of the chair when seated. If family members are larger in build, then add more to the inches. Next, when seated, the average person needs an additional 16 inches to push back the chair to get up. Again, add more if family members are larger than average. If all family members are of average size, then think about the guests who may not be; it’s best to be generous with measurements for everyone to be comfortable.
Also, look at your floor for duct vents. You do not want to cover any vents with the rug. Even with vents closed, they are weak when people step on them. Remember, ‘out of sight, out of mind’. You can’t expect even family members to remember there’s a vent under the rug.
Covering the vent will also cut down on the room’s air circulation. A table full of people seated, plus perhaps candles burning, generates a lot of heat. You’ll definitely want to keep the air flowing. Making sure everyone is comfortable with the room temperature, and enough personal space; this will make the entire dining experience more pleasant.
If you can’t make a rug work due to lack of space or floor vents, then don’t try to squeeze one into the area. Move on, and find other ways to bring warmth and color into your dining space. I’ve wanted a rug in my dining room for years, and unfortunately there’s not enough space without covering my vents.
***Area rugs can be used on top of carpet. Make sure you use a rug pad to keep the rug secure. There are rug pads to use on different floor surfaces; wood floor and carpets pads—only use the one appropriate for your area.