I'm a recently graduated Part II and I - and several others I know - are doing portfolios at A4 size. If you can use InDesign you can put together a really good portfolio by putting the pages portrait and then treating the two facing pages almost as one large A3 page. Added to this, an A4 portfolio may be cheaper to print, so you might want to print high quality two copies in case you're being interviewed by two or more people - that way they're not crowding around one copy and will feel more comfortable flicking through as you speak.

And don't forget - don't use too much text! More than three sentences or so on a spread and they won't even bother to read it. You don't need to explain everything in text - they have you there to explain anything they don't understand.

Good luck!