What size slow cooker do you use?

[quickname]

I was looking at slow cookers yesterday and choice seemed to be 3.5L (serves 1-4) for around £15 or 6L (serves 1-6) for £40+
We are 3.. me and 2 DC so a 3.5L (labelled as serving 1-4) would seem OK most days but then I started wondering if this would be big enough if we have a guest as we sometimes do or even to make extra and freeze.

Is it worth spending the extra for a bigger cooker? Have you found you need the biggest or is 3.5L adequate?

Mine is massive as I never make a meal for just the 3 of us - at least double quantities for lunch or freezing. So much easier!