The largest double-slit projectile I know of is C-60, a soccer-ball shaped molecule of sixty atoms.
https://iopscience.iop.org/article/10.1088/2058-7058/12/11/4
https://iopscience.iop.org/article/10.1088/2058-7058/12/11/4
> The largest entities for which the double-slit experiment has been performed were molecules that each comprised 2000 atoms (whose total mass was 25,000 atomic mass units).[19]