The massive screen was in Plaza San Martin at the base of the hill (former river bank) on the sidewalk just west-ish of the memorial.
Hundreds to thousands gather for each match with the largest crowds obviously being for Argentina's matches.
This year Argentina's first two group matches are on weekend days, but the third match starts at 1pm on a Wednesday. If you ever wanted to put down a blanket and have a picnic in the middle of Av 9 de Julio, that will be your chance. (Except for the bus lanes.) the city will absolutely come to a halt. We're talking London during a WWII air raid.
In the park you will be able to see precious Madonna & Child scenes like this: