180 will be perfect for three on one side but carefully consider table legs, if you are going with a contemporary design with the legs at the corner you will be fine, and this will fit 8 so room to grow :)

Funnily enough I looked into this in quite some detail recently. We also needed a new table, and in our situation, most of the time we are 4, but then we are a family of 6 for weekends and holidays, and every other weekend there's just 2 of us. Added to this, from time to time, when we have visitors we need the table to seat 8-10.

Having sat down at dozens of tables in-store, and having looked at probably a hundred on line(!), it seems that one should allow 50cm per person (excluding any table leg). You also need to bear in mind that you cannot sit people comfortably on the ends if there are people going right to the corner on both sides.

So generally I think a 150cm table is insufficient for regularly seating 6 but 160cm is ok (2 each side and 1 on each end). That said, I think 180cm is preferable (you can sit with room to spare with 2 each side; or 3 each side). 180cm also allows 10 if the table legs are well located on or near the corners, or the legs are central (e.g. like pedestal legs). Because you can have 3 on each side and 2 on the ends.

With longer tables, watch out for table legs that have a support beam running between the legs close to the floor. I found that those can prevent people sitting comfortably on the ends, because they cannot pull their chair legs under the table. The supports are also a nuisance if you are pushing end chairs under the table when not in use.

If you decide to look at extending tables, consider carefully the way that the table extends, there are half a dozen styles of extension with various pros and cons.

We eventually went for a 180cm table, that extends to 220cm. It has a "split-frame" extension so that table frame extends (not just the table top). This means the legs always stay on the corners (nobody has to sit with table legs between their legs) and the 40 cm extension leaf is stored in the centre of the table.

Thank you both for your very useful comments and advice! Unfortunately no room for a table to extend but glad to know that 160 should fit 6 on each side even if it's a bit at a squeeze. We will be able to shift furniture a bit and accommodate 6 easily (2 at each end) when guests with kids turn up so will go with that.

It saves some cash and space - especially when 80+% of the time we'll be with 3.

Cheers!