What I'd like to do is to be able to set it so the amount of text displayed in the columns is fixed and when the grid is resized the size of the font changes to keep the text displayed in each row fixed while having the number of rows shown vary as needed.
Using your Binding to 1 Million records Sample to show what I want, I took a screenshot stretched the grid by 50% in height and width to simulate text enlarging
as the window was widened, and then cropped out 5.5 of the 17 rows of data so that the number of rows visible was reduced because the height wasn't changed.
https://i.imgur.com/sctAk9F.png