58mm is the best size, you can buy it here from Home Coffee Machines Ltd in Ireland. They deliver free to Ireland and will deliver to Europe but there is a small charge.
It has a beautiful wooden handle and is weighted perfectly. A quality piece of equipment, but you would expect nothing else from Rancilio!
Some users have reported a 58mm tamper being too big, this issue will occur if you are not putting enough coffee grinds in your double portafiler and the tamper protrudes too far into the portafilter when tamping and makes contact with the sides of the portafilter.