Hi there, without a point if reference it's hard to say exactly but it looks like there is about an inch around the edge of each tier on the top one so say a 6", 8", 10", 12" and maybe an inch and a half around the one below. That said, the tiers on the second one aren't as tall so it could be a bit of an optical illusion so could still be an inch.
It's funny, my first wedding cake was for a friend and she showed me that top picture as her inspiration! I used those proportions and the tall tiers and one thing I can say is that it is a LOT of cake. Have fun!