Assuming you don't want to break the station in to separate stations and use a load of feeder links, you will need to tweak the advanced option that limits your maximum station spread.
This advanced setting (specifically
Max station spread) can be adjusted to values between 4 and 64, with a default of 12, and determines the maximum size square that can be allowed to bound each station (whether compound or not).
The recommendation is to only increase this if you are using the YAPF pathfinder, but this is the default so if you've never tweaked that setting there's no need to worry.
For more information, you can see the wiki entries for the setting and station spread.
If you need a station larger than 64 by 64, well then I think you're stuck (and possibly doing it wrong).