These should give you a general idea:
Potatoes, chips, frozen, oven, 100g cooked 4
Potatoes, chips, fish & chip shop, 227g serving 17½
Potatoes, chips, frozen, microwave, 100g cooked 7
Fish & Chip Shops, Chips, average portion, 227g standard 17½
Fish & Chip Shops, Chips, average portion, 340g large 26
Potatoes, chips, frozen, thin cut, deep fried, 100g 14
Potatoes, chips, frozen, straight cut, deep fried, 100g 9½
Potatoes, chips, homemade, average thickness, deep fried, 100g 5
The average size of one medium potato is about 250g, so I'd guess around 15 syns upwards for a regular portion.