Flask size depends on which synth you are doing, and by what method. The drier push-pull type rP/I2 reaction requires larger flasks because of its tendency to double and expand out of the flask with intense bubble formation;
the long wet reflux rP/I2 does not share those characteristics and can be safely done in a smaller flask.
Shorty is most familiar with hypo reactions done with some form of reflux, and these may be done safely in a smaller flask. I would prefer a 500ml flask for a 30 gm reaction. I tend to wash the reaction fluid and base it in the flask in this synth and the phos acid synth, and prefer to have more room in the flask to do this.
The Phosphorous Acid synth can be done in a flask similar to that used in the hypo reaction, particularly if the HI is made before adding the pseudo or ephedrine. This synth producea HI very quickly and become very vigorous at that point. A full flask increases the risk of balloon failure or seals leaking at the time one least wants a problem. Aside from that moment, this synth is almost bucolic.