Something

struct blah {
    int a;
    int b;
};
int
main(int argc, char *argv[]) {
    printf("sizeof(int)=%d sizeof(blah)=%d\n",sizeof(int), sizeof(struct blah));
}

.:peppar:. stric:/tmp>gcc -m32 a.c;./a.out
sizeof(int)=4 sizeof(blah)=8
.:peppar:. stric:/tmp>gcc -m64 a.c ; ./a.out
sizeof(int)=4 sizeof(blah)=8