The postures for Mass are given in the General Instruction for the Roman Missal n. 43. Here is my extract/summary of what it says for this part of the Mass:
“43. The faithful should stand … from the invitation, Orate, fratres (Pray, brethren), before the Prayer over the Offerings until the end of Mass, except at the places indicated here below.
The faithful should sit, … ; and, if appropriate, during the period of sacred silence after Communion.
They should kneel, on the other hand, at the Consecration, except when prevented on occasion by ill health, or for reasons of lack of space, of the large number of people present, or for another reasonable cause. However, those who do not kneel ought to make a profound bow when the Priest genuflects after the Consecration.
It is for the Conference of Bishops, in accordance with the norm of law, to adapt the gestures and bodily postures … Where it is the practice for the people to remain kneeling after the Sanctus (Holy, Holy, Holy) until the end of the Eucharistic Prayer and before Communion when the Priest says Ecce Agnus Dei (Behold the Lamb of God), it is laudable for this practice to be retained.”