What is a good size for a kitchen island?

This is entirely dependant on the size of your kitchen - it's not a case of one size fits all, unfortunately. If you have a small kitchen then you should look for a smaller island that can perhaps sit in a corner and be wheeled into the center of the room for occasional use. A meter between the island and your worktop/cabinets is considered a good amount of space - without it feeling too tight. If you are using the island as a breakfast bar then you should allow around 60cm space per person. Always measure your space before you buy a kitchen island.

What is the best countertop for a kitchen island table?

This largely depends on its main usage. There are lots of different countertops available for kitchen islands. Granite makes a good choice as it's hard-wearing - but can be expensive. A wooden butcher block makes a very popular choice and can be easy to maintain - but can pick up scratches and stains quite easily. Stainless steel is very practical and super hygienic, it won't stain but can dent over time.