Confusing about the product backlog sizing.

So my question is which means the developers? the developer is the development team or just the developer who works on the backlog. 

The Scrum Guide says:

Developers are the people in the Scrum Team that are committed to creating any aspect of a usable Increment each Sprint.

There is no mention of a separate "Development Team" in the present version of the Scrum Guide, and it speaks of Developers in the plural since this describes a joint accountability.

Any person in the Scrum team who is accountable for creating any aspect of the product increment (and the plan to achieve it) is considered a developer. 



This may also at times include the PO or SM, as it's an accountability, not a role. 



However if the PO or SM are NOT developers, and are NOT directly involved in the creation of the product increment, then their input into the size of backlog items is meaningless. 



Hope this helps!

Try to use a whole team, not just one person.  The idea is to get multiple opinions and minds into the mix.

BTW, story points are good for having conversations about complexity, effort, risk, and other assumptions, but not very good for forecasting or estimating when something will be done.  It's better to use flow metrics - look into scrumorg's kanban posts.