The default playable area on a Cities Skylines map is 9 tiles (3×3 with any adjacent tile being unlockable) but you can go bigger thanks to a mod that’s been created in the Steam Workshop which will unlock a total of 25 tiles on a map which makes the playable area so much larger.
The mod is available now in the SteamWorkshop and to get it working in the game you first of all need to subscribe to the mod (click the little tick on the image) which will add it to your Steam. Currently the mod downloads to an incorrect location so what you need to do is read on and follow the instructions…
The file will be downloaded to Steam > steamapps > workshop > content > 255710 > 403798625 as AllSpacesUnlockable.dll
You need to copy the AllSpacesUnlockable.dll file into the mods directory at Steam > steamapps > common > Cities_Skylines > Files > Mods
Now create a directory in there called AllSpacesUnlockable and paste in the AllSpacesUnlockable.dll.
To activate the mod, go in-game and click the Content Manager button. Go to the mods tab and check the 25 Spaces Can Unlock the full map mod.
The mod will now be active and you can now expand out beyond the standard settings. Here’s a few shots of how it looks.