IIRC, rubber "bullets" are somewhere around 30mm, which isn't that far off from the size of the rounds grenade launchers commonly use - I think those are usually 30-40mm. I saw somebody recently say that they're the size of 8 or even 4-gauge shotgun slugs, and an 8-gauge is 25% larger than a 12-gauge.
They're also not rubber like people think of when they hear the name. They're a metal slug wrapped in a layer of rubber or foam.
I saw somebody say that older rounds were designed to be used that way while newer rounds are meant to be aimed at center mass (both from at least a certain distance away to let the energy dissipate before it hits somebody/something), but cops have both kinds in their arsenal and fire them both directly at people from point-blank range - breaking all the rules for their use.