Buy the largest you can afford.
Especially for pastry one needs a large processor. Medium size will usually only handle one crust. It is such a waste of time being limited to a single crust.
Remember the processors look big but you can only fill them one third with liquid before it overflows down the centre.
I bought a mid-size (the motor was the sweet spot in the range) and really regret not having the largest. Also if you are planning at using it for chopping make sure it has a wide mouth. Then you don’t have to halve everything first.
A stand mixer in my view is more efficient for mixing cakes and bread dough. Processors are superb for chopping, grating etc.