Question about "a large fries"....

Yes, fries are plural....but when you are talking about an order ... 1 order .... 1 order of fries
1 large order of fries....  "a" large order of fries... or "a" large fries

a large fries = 1 order of large fries

Then you could order fries this way:
"I would like large fries, please"
"I would like 1 large fries, please"
"I would like a large fries, please"
"I would like 2 large fries, please"

The same rule would apply for ordering things that are uncountable....
Coffee / tea / juice / beer etc. (drinks) are normally uncountable:
I don't like coffee very much.

But you can say a coffee (=a cup of coffeee), two coffees(=2 cups of coffeee, etc when you are ordering at a restaurant.
I would like 2 coffees and an orange juice, please.

I found one video on Youtube.  If you listen carefully ... She says "a large fries" (=a large order of fries)

Hope that helps...Gotta run back to class now....Good Luck!  Everybody study hard!
http://www.youtube.com/watch?v=aA5CCpmDhkY&feature=related