Lot of discussions have involved this fucking island in the past. Now I'm here to gather more information than anyone else has ever provided here that I'm aware of.
I'll provide the evidence presented in the manga and I'll compare them to see if the size it has been discussed before is or not factually accurate.
In this first scan we see Vivi stating that in order to arrive to Yuba they have to walk from Elumalu. Such trip would take around half a day for them.
Second scan
First scan
Average walking speed is 1.36m/s.
12 hours goes about 43200 seconds.
Why don't I count the rest time? Simple, the statement is about walking to reach Yuba. When they rest, they don't walk, so the that doesn't count in the travel.
Distance = Speed * Time.
D = 1.36m/s * 43200s
D = 58752 or 58.752km.
In the second scan the statement goes about the distance between Yuba and Rainbase, the city where Crocodile's hideout is.
This time the statement is about a whole day of walking to reach it from Yuba.
D = 1.36m/s * 86400s
D = 117504m or 117.504km.
So with these two scans we have over 175km from Elumalu to Rainbase.
Third scan shows Crocodile stating that Vivi has around 8 hours left to go stop the rebels. Worth noting that they already decided to attack Alubarna at this point, so I'll assume they left Katorea around that time.
Third scan
They were riding camels to cross the desert. Some rebels were riding horses, but those are so low in number that they are irrelevant.
A camel can reach around 65km/h at peak and 40km/h to run more time. I'll use the low end. They were starting a war, but camels can't hold that speed for much time anyway.
D = 11.11m/s * 28800s
D = 320000m or 320km.
Fourth scan
Fourth scan happens before the infamous 50km wide river is revealed, yet right before that it's stated to be as wide as a sea. This is obviously an unusable statement, since seas can vary in size a lot, but this supports the idea that the river is wide.
Fifth scan
To cross this gigantic river they got help from the Kung Fu Dugongs (fifth scan), that at the very least should be as physically capable as their IRL counterparts, beasts that can swim at 10km/h and 22km/h in short bursts.
These animals are obviously much more intelligent and powerful than their nerdy IRL cousins, so it's not strange that a bunch of them were able to drive the crew through the river while carrying a giant beast in a single hour. Let's not forget these things kicked Usopp's butt who in fact has been keeping up with some Baroque Works agents.
Sixth scan
Smoker was right when he said this is a pretty big country (sixth scan), because this isn't even half of the island at all.
Seventh scan
If you paid attention to the scans where "maps" are shown, you'd have noticed that the main points of interest are indeed around the river and that small island in the left bottom side of the draw. In the last scan you'll notice where that is (seventh scan).
The panel doesn't show us where the capital exactly is, but to argue that it's around half of the island would require a lot of evidence to counter the scans I've posted.
It turns out that one of the major threats Croco-boy did to Vivi was the goddamned bomb that couldn't kill a glorified oversized pidgeon.
Alubarna's City Square is around 5km wide (diameter). So going by this idea, the square goes around the same size, giving us a way to scale the whole city.
11px = 5km.
53px = 24.09km.
If you look closer to the seventh scan, you'll notice that the river (50km) is around 2-3px at best, while neither Alubarna nor the other cities can be seen. It's obvious, since they'd be a small spot on the draw with no clarification, showing that the river is actually this wide, therefore, the island is as big as we thought.
Insert your "Oda just went crazy" comment down below, but with arguments. Because while it's true that some distances don't add up, it's way more consistent than people thought it was, dismissing the river's width because it was too big to be real.
