Absolute minimum circumference per player is 22in (55smth cm) or 24in ideally (61cm). Total circumference of a circle is 3.14x the diameter.
For 8 players (8x let's say 60 =480cm), the table's diameter should be in the area of 60in (152cm) or 155cm (61in) - absolute max 160cm (63iin).
Beyond that, the table becomes too large for most rooms and, more importantly, the middle of the table becomes impossible for anybody to reach.
So, for 9 people or more, you have to switch to an oval, which, for self-dealing purposes at least, should be as round-ish (short and wide) as possible.
The circle, as a shape, is ideal for both communicating with other people and any for any sort of game, but it takes up more space, for a given circumference, than other shapes.