If you just want to split up a big sketch, you can split it all into tabs with the .ino extension. The IDE will paste them all together into one big .ino file before compiling and it will compile just as if they were in one file.
If you have an object class 'X' that you might eventually turn into a library (for use in multiple sketches), make an X.h tab for the declaration and an X.cpp tab for the functions too big to easily put in the class declaration. Then #include "X.h" in your main sketch. The X.h file will allow you to use the X class in your sketch. The X.cpp file will be compiled separately from you sketch and the functions you call will be added to your compiled sketch. Later, move the files to libraries/X/X.h and libraries/X/X.cpp and the #include "X.h" should still work.