My measurement is about 1/4 inch square, accounting for compression and age. 1969 and 1970 Dearborn Cougar examples. 1969 is right along the lower edge of the glass, 1970 has a distinct gap. I know, two examples is not a trend.
I can see two benefits for this piece of foam:
1. It prevents the lower windshield trim from rattling.
2. It acts as a dam to prevent water from going directly into the cowl.