where ... means you fill in the other 997 or so.
size_t answer = sizeof(structure1) + sizeof(structure2) + sizeof(structure3) ...
That's absolutely insane to have a header file containing 1000 structures.
Why on earth are you being asked to do this ridiculous thing?
What is supposed to be done if there are structures inside structures? Surely adding the size of the inner structure individually only makes sense when there is no instance of that struct contained within the outer struct. This exercise reeks of stupidity.