There are multiple techniques can be adopted to support the PO during backlog refinement session to keep the PBI well-refined. However, irrespective of any techniques, the main goal is prioritisation should be in such way that would deliver maximum business value by end of upcoming sprint. Here's few on how to achieve that.
MoSCoW technique : The stories can be prioritised based on importance of functionalities (Must have, Should have, Could have, Would have)
Regarding the defining the stories, it should meet INVEST criteria (Independent, Negotiable, Valuable, Estimable, Short, Testable)
Regarding the sizing, the bigger stories should be splitting into small stories based on Workflow, user role, business rules etc. We can use T-shirt sizing for epic level stories to further break it down.