[Sora@SoraLaptop src] $ cat size-test.c
#include <stdio.h>
#include "lua.h"
#include "lauxlib.h"
#include "lualib.h"

int main(void)
{
        lua_State *L = lua_open();

        printf("%d", L->stacksize);

        lua_close(L);

        return 0;
}
[Sora@SoraLaptop src] $ gcc size-test.c -I. -L. -lliblua.a -o size-test
size-test.c: In function ‘main’:
size-test.c:8:17: warning: initialization makes pointer from integer without a cast [enabled by default]
  lua_State *L = lua_open();
                 ^
size-test.c:10:16: error: dereferencing pointer to incomplete type
  printf("%d", L->stacksize);
                ^
[Sora@SoraLaptop src] $