The best shape depends on your space and how your household gathers. Round outdoor dining tables are a good place to start, because they have no corners, so talk flows, and everyone can see each other. That easy feel makes them work well in a tighter spot, and if you like the look but want a bit more length, an oval does the same job.
When you are regularly feeding a crowd, rectangular tables make more sense, since the straight sides seat more people and suit longer decks. Square tables take the opposite approach, sitting four close together in a way that feels balanced and intimate in a compact courtyard.
If your numbers change week to week, an extendable outdoor dining table bridges both, giving you an everyday size that opens up when guests arrive.