“Again, that's really far-fetched.
The map of Alabasta clearly shows a big river, but it's nowhere near as big as you describe it. It was described as "taking a day to float across", which isn't really that unbelievable, considering floating is damn slow.
My guess would be it's around the same size as Iceland, but nowhere near the size of Australia. (It's actually a little bit larger than that)
Also, in chapter 214 you can see the route took to escape Alabasta. They covered that distance within a day, which would be impossible if it was the size of Australia.
Actually, let's look at another example, Crocodile mentioned it would take the rebel army 8 hours to make it to Alubarna from Nanohana. Let's say they average out at around 30 km/h (Fairly realistic for long distance horse travel). That would mean they had covered 240 kilometers within that timespan. Based on the map from chapter 214, that's around 1000 km from shore to shore on the long side. Which is around the size of Indonesia's largest island.
Overall it's fairly inconsistent on Oda's part. The Straw Hats covered atleast 1000km within 12-15 hours or so on a ship, where the rebels only covered 240km in 8 hours”
I find this from a old Reddit Post
I can’t completely vouch for this post regarding Alabasta’s size at least since the links are unfortunately to online piracy sites except this one.
Here is the map for Alabasta