I'll be using two different methods to try and figure out the size of Thriller Bark.
The first one will compare the height of Oars to the size of Thriller Bark directly.
The second will find the width of the main tower on Thriller Bark and the distance from the tower to the coast.
Method 1: Oars and Thriller Bark
This method is fairly simple, we just compare the height of Oars to the width of the ship.
Oars Height = 13 px = 67 m
Thriller Bark (Land) = 326 px = 1680.15 m
Thriller Bark (Total) = 379 px = 1953.31 m
Method 2: Radius of Thriller Bark
Step 1: Door Height
First we need to get the height of the door to Thriller Bark's main tower which we can get from Zoro.
Zoro's Height = 22 px = 178 cm
Door's Height = 216 px = 1747.6 cm
Then we can find out the distance from this door to the coast of Thriller Bark by comparing it to a panel of Franky and angsizing the distance:
Step 2: Tower to the coast
Franky's Height = 21 px = 225 cm
Door's Height = 31 px = 1747.6 cm
Panel's Height = 977 px
2atan(tan(70/2)*(21/977)) = 0.0203679941 rad = 1.16700009908026 degrees
2atan(tan(70/2)*(31/977)) = 0.0300658134 rad = 1.72264421545215 degrees
Put that into the angsizing calculator and:
Franky's distance from the screen = 110.46 m
Door's distance from the screen = 581.09 m
Franky (at the coast) to the door = 470.63 m
Now all we need is the width of the tower and we can figure out the diameter of Thriller Bark.
Step 3: Width of the Tower
Using the height of the door we already calced:
Door's Height = 22 px = 17.476 m
Tower width = 205 px = 162.84 m
Let's add that to the diameter of the island we already calced and we get:
Thriller Bark Diameter = 1104.1 m
And that's just for the land of the Thriller Bark, not counting the bit of extra water see going from the land of Thriller Bark to the wooden ships wall.
Method 1: Width of Thriller Bark's land = 1680.15 m
Method 1: Width of Thriller Bark = 1953.31 m
Method 2: Width of Thriller Bark's land = 1104.1 m