there is no perfect dimensions for an island. Try to organce space subdividing into smaller cabinets of 60x60 cms which is the max space for a structural division and works for seating area as well

You should be able to get everything that you want ........ Are you going to have a cabinet maker build it for you ? I suggest that you work with them and have them draw it up for you........Most cabinet makers are very good at working with their clients to get them what they want.

Option 4. I would not have more than 2 different cabinet finishes. Darker, grounded look for the base and island, with lighter, airier look for the uppers would balance the room.

I have used both with good results. The main difference between Quartz and Granite are 1) price, 2) colour range. Granite generally works out cheaper, because you can buy the exact quantity you require. Quartz comes in large slabs of specific sizes- standard size being roughly 10'-0" x 4'6", so there tends to be more wastage if you don't plan the cutting well. But Quartz is pretty and comes in a wide range of colours and can be re-buffed at a later date. Quartz also comes in various thicknesses- the most commonly used being .75" to about 1.25". And especially when you use dark wood cabinets in a kitchen or bathroom, beige and white counter tops work very well. Some people go even with coloured counter tops like blue, orange, yellow. Natural stone in light colours tend to be porous and stain easily, so I prefer to go with pure black which is safe.