I was asked to make this wedding cake exactly like the picture and I've been looking at it trying to figure out the sizes of each layer. I'm guessing 4" 8" 10" 12" 16", but not sure. Any/all help is greatly appreciated
It would depend upon how many servings were required. The tiers appear to be two or three inches different in size, but the same from top to bottom (if that makes sense). Example: 12", 10", 8", 6", 4" or 15", 12", 9", 6", 3"
The difference in the height of the tiers, I believe, give it the dramatic look, not the diameter.
Personally, I would not want the double barrel tier in the middle to be any larger than 10 inches because it would just be too heavy for me to handle. Also, it will require an enormous area for rolling out the fondant. I've only done one that large and I used the technique where the fondant is wrapped around the sides. You could always have a sheet cake ready for serving, if you were a few short on servings.
Here is Wilton's serving chart. I always use the "party" servings to be safe. Remember that your double barrel will feed twice as many as shown on the chart and the really short tiers will probably do about half as many as shown.