You honestly would not believe how hard it is to find the perfect divers SMB. Either you can only orally inflate them like the Dive Rite See Me marker, or you can only inflate them with your Octopus and they don't have a funnel valve at the bottom to stop them deflating once they get to the surface. And if you can find one that offers both of these features then it is that large that you can't actually take any other diving equipment as it fills your dive bag!
So we were very happy when Mares have re-introduced the Mares Divers SMB - we know it is expensive but it is the only one we have found on the market that has all of the functions you would want from an SMB whilst still remaining relatively small and lightweight.
You can orally inflate it, fill it from your Octopus, it has an overinflate release valve and is not much larger than a conventional SMB.