I have a 6 lt and a 9 lt one (both Tefal, also known as SEB). The smaller one lives and is used in campervan or on boat, big enough for meals
for 4 people.
a new one, check for diameter and how that fits on your stove. Generally higher is better, as it also allows the of use stackable trivets.
Note: size is generally marketed with the pot filled to the rim, but when used in cooking
one has to stay ~5 cm (ie up to 2 inches) below that. Again that favours narrow and high pots as opposed to fat and low pots.
Of course the 3 main advantages are:
* shorter cooking time
* therefore minimising gas or fuel
* minimise dishes