You should account for at least 60 cm in length per person at the table side. A table also has two end sides. Assume you place one person at each end and four people on the long sides (2+2). Then the table needs to be at least 120+60 cm long to accommodate the plates for those sitting at the ends. But then you might as well place three people on each long side since the table would be 180 cm long. You could then make it narrower instead. At knee height, it should not be narrower than 70 cm so the knees won't hit the person sitting opposite. The tabletop can be a bit wider, say 80 cm. The dimensions would then be 80x180 cm. If you want end placement, you should allow for 100 cm in width. In return, you can then fit eight people around the table.
Then it depends on how you want to make the table. Since it should stand outdoors, both pressure-treated wood and other materials might be suitable, such as glass and aluminum/stainless steel.
Assume pressure-treated wood, class AB according to the NTR method. (A would suffice since it probably doesn't stand on soil?) Four legs of 70x70. A frame between them of 45x70, tenoned into the legs. The tenons are locked with a 12 mm dowel from above, centered on the junction between tenon and mortise. One tenon per side. A cross brace in the middle of the long side of 45x70, tightly placed and fixed with a 12 mm wooden dowel and glue to the long side frames. Seven planks at least 22 mm thick, preferably 28 mm, and 120 mm wide, with a 10 mm gap between each. The total width then becomes 90 cm, which could suffice for end placement.
If you are making it from glass and aluminum (or stainless steel), there are ready-made parts to buy and the glass top can be ordered from the nearest glazier.
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