How to make an Overworld tile bigger?

Okay, as much as I like my Overworld map, I have a problem with one of the tiles...


You see that whirlpool? I wanna make that bigger! That's only one tile, but I want it to be like 4 tiles of space. The reason for this is because I want there to be a point in the story where the characters's ship gets caught in a nasty storm and sucked down into a big whirlpool which breaks apart their ship and they wake up in an underwater city and yadda yadda yadda.


That tile though? It's tiny!

Yes, it is tiny!


I suggest taking that tile, putting into Photoshop or Gimp, then resizing there and pasting that back into the sheet where you want it (or a new sheet)

How do I do that? When I load it into GIMP, I get this (first pic) while the actual in-game tiles look like they do on the left of my map (second pic). And considering that I have a high resolution monitor (3840x2160), sometimes I have trouble telling what exactly something is and it makes the interface at the top smaller than what I see in other screenshots and videos.


For example, what are those yellow plants between the log and the crate? What's those black helmet looking things next to the double chimney? What's the thing made out of wood below the barrel and above the pine tree? And what's the thing beside it? Looks like a place to put a saddle.