(Sorry if this explanation suck, english is not my mother language)
Over the course of Alabasta Arc it was imply multiple times over that the bomb was only meant to destroy Alubarna's City Square the same place where both royal army and rebel army were fighting each other, a bomb which would had generate an explosion capable to kill everyone within a blast diameter of 5 km.
This mean the City Square must be around the same size of the blast, since if it wasn't the case then Crocodile would had no reason to trick both armies to fight inside the City Square as he planned, like if the bomb would had completely wipe out the capital then Crocodile would had left the fight happen even in the entrance of the city.
This, like i had stated before, was confermed more than one time over of the arc, meaning this isn't just one random quote take out of context.
An a major evidence of the City Square is that it was vast enough to fit both royal and rebel armies, this is important because the armies put together had more than two million fighters/soliders, more precisely 2,000,000 rebels and 300,000 royalists, all this amount of people were supposed to be inside the City Square fighting each other, which its evidence of the City Square to be massive.
However, if we are unsure about the City Square be that large despire the many statements, it is still possible to find a Low End by using the scan below:
Zoro stated that the bomb was going to destroy most of Alubarna, meaning that the explosion and the destruction generated would had be still be not enough to destroy the city.
Y = ((x/0.28)^3)/1000.
x = 5 kilometers in diameter or 2.5 kilometers in radius.
((2.5/0.28)^3)/1000 = 0.711780248 Megatons or 711.780248 Kilotons.
Difference between near-total fatalities and widespread destruction (by using 1 Megaton as example):
7.2 km (widespread destruction) - 2.7 km (near-total fatalities)
7.2/2.7 = 2.66666667
2.5*2.66666667 = 6.66666668*2 = 13.3333334 km in diameter
Alubarna's diameter = 13.3333334 kilometers.
By using the parameters i had found in my previous blog, linked here.
600 pixels = Alubarna's Diameter.
128 pixels = City Square.
128/600 = 0.213333333.
13.3333334*0.213333333 = 2.84444445 kilometers, a little more than half 5 kilometers.
After thinking about i estimated that the scaling i have overcomplicated the calculation.
Due of this i will redone it completely by using a more simple and accurate method, instead of drawn a circle for find the diameter of Alubarna i will just use one single line (since Alubarna is a city with a shape of a circle).356 pixels (Black line, Alubarna radius) = 6.66666668 km
91 pixels (Blue line, Royal Palace full length) = 6.66666668*(91/356) = 1.70411985 km
137 pixels (Red line, City Square diameter) = 6.66666668*(137/356) = 2.56554308 km
Less impressive but it still pretty good.