So since the A4 is growing in size, will the next A3 also be a bit larger?

Probably not for it's market, weight reduction would be welcome...

I would have to guess yes
 

The A3 is basically Audi's version of the VW Golf and the Golf has grown in size every model change. Usually, a car model will grow in size slowly until it's gotten so big that car companies come out with another brand new smaller model to fill that old niche.

I remember reading about how the original Honda Accord used to be smaller than the Honda Fit!

And yes, weight reduction would be very welcome.

is it growing that much? or is the wheel-base being shifted (to be more like a bm-dub)