I have discovered that when drawn to scale, you can fit Khorvaire, Sarlona, Argonnessen, Xen-drik, Aerenal, Frostfell, Everice, Faerun, the Flaneass, Ansalon, Taladas, a good chunk of Athas, and Maztica in one world,
the exact same size as Earth, and have it make coherent sense, without overcrowding with continents. Here's how:
1. Eberron makes up about 50% of the map. Place it on the left.
2. I wrote a computer program to convert a scan of the Frostfell map from Player's Guide to Eberron from polar conic to equirectangular centered on the equator. It worked pretty well after a little tweaking the output image, although I lost the details near the north pole.
3. Place Faerun and Oerth back to back (requires a very slight adjustment to the north coastline) to the east of Sarlona. Replace the tiny island north of Sarlona with Evermeet.
4. Connect Ansalon to the Everice southeast of Argonnessen, and place Taladas southeast of Sarlona and northeast of Ansalon, along the southern tropic line. Notice the proximity of dragons in Argonnessen and Ansalon. Also notice how tiny Ansalon and Taladas are when drawn to scale against the other continents.
5. Place Athas along the equator about where Zakhara normally is. Notice the Plains of Dust merge nicely into the sea of silt. Merge the Kreen empire on the Crimson Savannah with the northwesternmost tip of Zakhara (I only used the section of Zakhara from the Corsairs of the Great Sea boxed set). You have to adjust Zakhara ever so slightly to be a little closer to Faerun but it's barely noticable. Place mountains along the east, southeast, south, and west borders. The Dead Lands and Anattan Wastes are huge so Athas takes up a lot of space.
6. Fuse Maztica into Hepamonaland. Notice the similar architecture between Maztica and the giants of Xen-drik.
7. Now place Ravenloft on a little peninsula east of Taladas. Ravenloft is really super tiny so it can fit just about anywhere.
8. Nehwon is kind of cold so place it somewhere to the south of Athas around 50 degrees south latitude. It's about the same size as Ravenloft.
I had enough room left over on the map that I designated them for three home campaigns - one is my original home campaign, one is a d20 Apocalypse (the aplocalypse was 25000 years ago, progress level 8, think Expedition to the Barrier Peaks), and an oriental adventures campaign a la Kara-Tur. I'd post the map, but it's mostly WotC maps shrunk down to fit on a 4096x2048 equirectangular projection (albeit maps that were downloaded from the WotC site for free, except Faerun and Maztica from the FR Interactive Atlas globe), and I don't want lawyers to come knocking.
So, what does this imply? The Dreaming Dark is infiltrating Faerun too, Cordell never came to Maztica (or came from Greyhawk), and Ansalon and Taladas have different climate zones than a normal Krynn campaign. The dragons of Argonnessen and the dragons of Ansalon have a mysterious connection. Minotaurs of Taladas raid Ravenloft for slaves and sell them to the yuan-ti of Faerun.