There’s a priest downtown who makes a habit out of extravagantly extending his hands at Mass. So high does he reach them that you fear he might spin into a pirouette. Fr. Edward McNamara, the house liturgist for Zenit, takes up the question of what’s appropriate.
Here’s a snippet:
The extraordinary form is much more specific. As one popular ceremonies book describes the gesture at the collect: “While [the priest] says ‘oremus’ he extends the hands and joins them again, and he bows his head to the missal. Then he reads the collect, holding the hands uplifted — but not exceeding the height or width of the shoulders — and extended, the fingers held close together and bowing towards the missal should the name of the saint in whose honor the Mass is celebrated occur. When he says ‘Per Dominum nostrum’ etc., he joins his hands.”
While a priest celebrating the ordinary form may not be strictly bound to these exact norms, I would say that they do provide a good rule of thumb as to what the Church understands when it asks priests to pray with hands extended. These rules were not invented by some obscure 16th-century curial official but are rather the codification of an already existing custom that had developed over several centuries.
A priest could follow the above rule. However, since the post-conciliar liturgy deliberately left out a strict specification of the gesture, it is also legitimate to extend the hands a little further if he considers it appropriate. For example, some modern vestments tend to require a somewhat more ample gesture than the traditional Roman chasuble. The above rule, however, does caution against exaggerated gestures that tend to draw attention toward the celebrant himself and not the prayer he is reciting.