You can create a single key for the post and then encrypt that key using the public keys of the authorized readers. With 256 bit keys this is 32 KB of keying.
But still the problem remains that the posts after already being decrypted will be reposted or leaked.
Yep, I don't see how it can work. The more followers, the bigger the risk of weak "links" that could leak the info.
Plus all encrypted blockchained info are just running on a ticking clock till their decryption. I think Satoshi first said that you don't want any kind of encrypted messages in the blockchain - it would be an accident waiting to happen...