Depends what you mean by "size". Conventional junction boxes are usually described by their current carrying capacity - so you'd select one appropriate to the current carrying capacity of the cables you were joining. In practice though it's common to over-size a little as the larger sizes provide more wiring space and so makes life easier for the electrician at minimal extra cost - e.g. a 20A joint box on a 6A lighting circuit or a 32A joint box on a ring final circuit where the cables are only rated at 20A.
Depends what you mean by "size". Conventional junction boxes are usually described by their current carrying capacity - so you'd select one appropriate to the current carrying capacity of the cables you were joining. In practice though it's common to over-size a little as the larger sizes provide more wiring space and so makes life easier for the electrician at minimal extra cost - e.g. a 20A joint box on a 6A lighting circuit or a 32A joint box on a ring final circuit where the cables are only rated at 20A.
