Pica's torso + head, is even above actually. But that one is a good estimation.
So Pica's Torso + Head is above 1,5km? Do you even know how much that is?
The highest building in our world is 829,8 meter tall and that's insane - and you are saying Pica's torso + head is more than twice than that...You trippin...
The corida coliseum is based on a real coliseum. It only has 1 extra layer. Even the size of the ''doors'' and windows is the same. You can see that on the people standing there - both in the real foto and in the manga.
The real one is 48 meter tall. Since the one in OP has 1 extra layer, let us add +12meter. Which means the corida coliseum is 60 meter tall.
The corida coliseum is roughly half as tall as the old royal plateau. That makes the plateau around 120meter.
Pica's whole body is roughly twice as tall as the old plateau - so around 240meter.
If Pica would be as tall as you say, then wtf are those marines shooting? Those are standard canons, but the explosion covers up a huge part of his head. That mean would that those canon balls create explosions of the size around 200meter, which is ridiculous.
Or look here. When Zoro cut up pica's arm, those pieces were around +100 meters ''thicc'' - based on your calculations - yet Pica's head is clearly seen when he is coming out. His whole body would take half off the space. And you can even see Zoro clearly, who is even behind the hand.