What is the recommended size for a Development Team (within the Scrum Team)?

What is the most appropriate action for the Scrum Master to take if the Product Owner is having difficulties managing the Product Backlog?

You have been hired as a Scrum Master for a company that has been doing business for over fifty years. In order to stay competitive, they have started an initiative to digitize their legacy systems. The company has several Scrum Teams working on different components that will be integrated to a single back office platform.
Your team is responsible for building the back office platform and integrating all other components. The Scrum Teams work in two week Sprints and are expected to deliver all functionality in six Sprints.
During development the requirement changes in the other components have been slowing down your team's progress. Because of these changes, your team has estimated that they will not be able to deliver all expected work within the original timeframe. The Scrum Teams working on the other components confirm that they are still on track to meet the expected delivery date. The program manager in charge of the digitization initiative is upset and angry with your team.
As a Scrum Master, what could you do to help the Product Owner?

Which approach would you recommend to ensure that the Product Backlog items discussed at Sprint Planning are well understood, actionable, and easier to forecast?

Which three statements best describe the purpose of having a Definition of Done? (Choose three.)