Well this made me laugh. Good job at that.
But since we are using ships to calculate the height of the walls to calculate half the size of Pica, all according to the opening post, why not use a ship whose height we know? AKA The thousand Sunny. Whose height has been stated to be 56 meters.
And following your example I added up the ship to a wall outside of DR.
there you have 6 thousand sunnies covering what would be like 60 percent of the wall, probably slightly less. So we can add 4 more to cover all the height of the rocks. So it can fit 10/11 sunny heights there.
10x56=560
11x56=616
And that following your example again would make half the height of Pica.
So 560x2=1120
616x2=1232
We can even say it will fit 9 which will get us a result of 1008 meters.
And before you pop that bullshit about marine ships, this is what a marine ship looks like compared to the Sunny
So there we have it. Following your own method, but just one correction, that is the staring point, because I used something we know and don't need to speculate upon, i.e. concrete measurements, we arrived at the conclusion that Pica is around 1000+ meters tall.