The minimum space per place setting
For a guest to eat comfortably, cut their food without knocking elbows with their neighbour and still have room for 3 glasses, a side plate and full cutlery, they need a minimum of 60 cm of table edge.
At fine-dining banquets with 4 or 5 glasses per person (water, white wine, red wine, cava and dessert), the ideal width rises to 65-70 cm per setting.
Round versus rectangular: what each one costs you
Round tables: every guest can see each other, which helps group conversation and the view towards the head table. They need more passage space around the perimeter.
Rectangular and imperial tables: very photogenic, they make the most of long rooms and rustic barns and allow continuous floral runners, but they limit conversation to the guests beside and opposite.
The cost of overloading a table
Forcing one extra person onto a round table — say 12 on a 180 cm table — drops the space per guest below 48 cm. Glasses start getting mixed up between neighbours, chairs collide when people try to stand, and table service slows down noticeably.