Is there any guarantee on the size of an std::reference_wrapper?

The C++ standard does not pose any size requirements. Per [refwrap]

1. reference_­wrapper<T> is a Cpp17CopyConstructible and Cpp17CopyAssignable wrapper around a reference to an object or function of type T.
2. reference_­wrapper<T> is a trivially copyable type.

All we know is that is copyable, and it is trivial. Other than that it it left up to the implementation. Typically it is just a wrapper for a T*, but the implementation might have some other members in there for some reason

There is no guarantee in the standard.

No sensible implementation is going to use more than a pointer.

static_assert it fits and you have enough alignment so your code is correct (and people can see it is correct). That static assert won't ever be hit.