31 cards is stall enough for me
On a serious note, around 40 seems to be the optimal number. any more than that is only consistent with persistent card drawing i.e. hourglasses.
The other option for a fatter stall is that the deck must be modular (more cards fill the same niche, for example dusk and turtle shield) since you cannot expect to reliably draw any card if your deck is bigger than ~40, so your strategy kinda falls apart.
what I often aim for with my stalls (if quanta allows) is to make it as small as possible and include a sole eternity(PA!) and a photon.
But then again, most stalls I design doesn't aim for winnig by deckout; but by winning in an uncounterable way. So it's natural that I design them as thin as possible, since they are basically a 'rushdeck' that gives up its early game for inevitalbility.