I understand the maximum size to be 20x20 cm - I bought a bag of 40 'small freezer bags' at Sainsbury's on Sunday for something like £1.69.
Coming home from the US last November, a very stern customs official told me she should confiscate my liquids as the ziplock bag was slightly too large (maybe 30cm x 20). Luckily she didn't, but I think there are some difficult ones around!