1200mm or 4 feet would be ideal.

Either will be nice, especially with a speckle through it, the black will be more dramatic, it depends what suits you the most, I myself would go for black as I like contrast and I'm a bit of a drama queen lol

Water marks don't need to be cleaned off (unless they are permanent). It just means your stone needs to be sealed. You can get a high grade sealer/ impregnator at Granquartz or Regent. Seal your marble twice a year and that problem should go away. Buy one of the half liters or 4 oz bottles (the former should last you two seals at least). This is a good quality for marble: https://www.granquartz.com/Pages/ItemProfile_Detail.aspx?ItemProfileId=1748&ItemClassNo=92 And this one is particularly good for sealing marble, especially in the kitchen: https://www.regentstoneproducts.com/item_detail2.aspx?ItemCode=ZETASEAL

So would something like this be ideal?