Derive Macro Size 

#[derive(Size)]

use object_rainbow::Size;

#[derive(Size)]
struct Three<A, B, C> {
    a: A,
    b: B,
    c: C,
}

object_rainbow::assert_impl!(
    impl<A, B, C> Size for Three<A, B, C>
    where
        A: Size<Size = typenum::U2>,
        B: Size<Size = typenum::U3>,
        C: Size<Size = typenum::U7>,
    {}
);

assert_eq!(Three::<[u8; 2], [u8; 3], [u8; 7]>::SIZE, 12);