1200mm or 4 feet would be ideal.

How about butcher block instead? IKEA and the big box stores sell it for countertops, the price is decent, and you won't have as many issues with buckling, water damage, or what to do with the edge as you would with the ply. If the edges of the ply get wet, or if any seam around a sink gets wet, the plywood will "blow" and you will have to replace it all, anyway.

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?