As a rule of thumb, each person needs 60 cm of table edge and about 40 cm of table depth for a place setting.
That is why a dining table for two rows of guests is normally 80 to 90 cm wide: two place settings plus room in the middle for dishes. At 60 cm per person there is room to eat without touching elbows; at 70 cm the table feels generous, and chairs with armrests need that much to pull in side by side.
The width of the chair matters less than you might think. Our slimmest chairs, Bella and Isabell, are 43 cm wide, and the gap they leave is what makes 60 cm comfortable. Sella and Erik, with armrests, are 50 cm and 54 cm wide and need the full 70 cm.