1. Who creates the Product Backlog estimates?
The people doing the work: The Development team which includes all those involved in getting the work Done.
2. Is it the Product Owner who determines how many product backlog itrms the Dev Team should select for a sprint?
It is a collaboration between the Product owner and the Dev team. The Dev team is best capable of forecasting the quantity of work. Sharing this with the PO will influence the grouping of work the Product Owner feels will best maximize the time. Consider it a friendly, mutually beneficial negotiation with the best interests of customers, Scrum Team, stakeholders, etc. in mind.
3. Who is responsible for removing new team member in a team, if necessary?
While we'd hope the Development team is capable of accomplishing this goal, the many maturities of team's and realities of chosen management structures often put this in other's hands. Be conscious of the impacts and choose appropriately.
4. Do items in the Product Backlog tend to be smaller than the items in the Sprint Backlog?
Quite the opposite. As Product Backlog Items (PBIs) get closer to the sprint they become more well known, decomposed and, finally, during Sprint planning and the Sprint turned into a plan of execution. You might think of it as an ever narrowing filter of understanding until the day the work is completed.