The team themselves are involved. That includes developers, testers, designers and the scrum master. If your backlog has been refined and prioritised, the team can make the decisions about what and how much need to go into the sprint.
I don't see the business (or customer, or client) in the list, but they certainly help determine the priority of the backlog/stories. From this priority (which can be fluid), we can then determine sprints.
Everyone that will be working on the sprint. They are the ones responsible and know what should be / can / can't be architected, designed, built and tested in the sprints time frame.