EDIT: Sounds like you want to work with DLL files. Which is not SDL specific but C specific when it comes to programming in SDL.
I always seperated my functions, variables, and DATA files into separate files and put them into *.h headers.
Then in my main.c I'd just do this
#include "data.h" #include "Variables.h" #include "Functions.h"
And if you're making a game it is going to be very large to begin with. It is all how you code and how much you are going to put into it. In (WOW)Wrath of the Lich King, there were approximately 5.5 million lines of code in the game, not including database entries and what not. I don't understand what you mean by your game will get big.