Does AUTHORIZATION_REVOCABLE flag affects all credits help or up to any amount? If one issues currency with AUTHORIZATION_REVOCABLE flag, and account A hold 100 credits of it, can issuer revoke only 13 credits? Can account A spend the rest 87 credits as normal?
No it isn't that fine grained. The credit will just be frozen until the user deals with the gateway and the gateway unfreezes.  The gateway could of course unfreeze the credit after 13 of it is sent back.