I would say approx one quarter of a plate. Whilst they say you can eat unlimited potatoes, I would advise some moderation. You won't lose much weight if you fill a plate three quarters full with chips. A quarter protein, a quarter carbs and half plate veg is what I aim for.
On the old Red plan a Hex B of potato (with skin) was 200g. I liked to spread my 2 HexBs over the day in half portions so would go for 100g - definitely helped with weight loss and 100g of potato is not that tiny an amount. 200g is plenty.
On EE it is just as long as speed is 1/3 so maybe 1/3 chips and 1/3 protein would be a good place to aim for.