Planning a dining room around the table

Start with the room, not the table. Leave at least 90 cm between the table edge and a wall or sideboard so chairs can slide back, and 120 cm where people walk behind seated guests.
How many seats
Allow 60 cm of table edge per person. A 200 cm table seats six comfortably, eight for a short dinner.
Light
Hang a pendant 65 to 75 cm above the table top, centred on the table rather than the room.