I'm going to cancel this question by answering it myself! (Using a row of 5 across)
I found out (by accident) that if I can complete one full row of product images, the dimensions propagate to all subsequent rows. I don't know how that's possible, but it's working for me.
It only resizes if the very first row is less than 5.
In other words, I was testing with 3 images. Flex would resize them to fill the whole row. But later on, I tested with 18, and the remaining 3 retained the same dimensions as all the others.
Unexpected behavior, but it's working for me!
Huh!