How would the seats in the proposed Parliamentary Assembly be allocated?

The precise allocation of seats in a Parliamentary Assembly will have to be determined by international negotiations. Unlike the composition of the UN General Assembly, a key feature of a Parliamentary Assembly should be that the number of delegates to be elected from the member states is graduated, with population size being an important criterion. However, to achieve a balance between the principles of democratic representation ("one person, one vote") on the one hand and the equality of states on the other ("one state, one vote"), the principle of degressive proportionality could be applied, as in the U.S. Congress and in the European Parliament. This means that the citizens of small countries would be relatively better represented per capita than those from large countries. Here and here you can find two publications that look into possible models.