I truly do not understand how there could be down votes on this.

well i dont know about dimensions, but you could do several different sizes,

for instance, at first glance, i'd say the path leading to the structure is about 4-blocks wide, but if you use that ratio for the whole thing, the tower wouldnt actually be all that large, probably reaching 14 levels or so.

another thing you could do is make the path 8 blocks wide, and use golden-blocks to represent the little torch-spots. that would be a good scale to work with

if you go with a 8 block wide path, the entrance would most likely be about 12 blocks tall, and you can base everything else off that.

So story short, make a ratio based off of the path.

the height of the doorway looks to be about 1.5 * width of the path, and each level of the structure seems to be the same height as the doorway.