I use these from Lakeland
Zip-Seal Freezer Bags - Lakeland, the home of creative kitchenware
They hold 1.5 litres (I've just checked with a jug of water). Nobody's complained it's too big, but I don't put a lot in it. They look about the height and width as the ones that were being handed out at Heathrow when I went through earlier this month.