Can I set default estimates for new backlog items and tasks?

Yes, you can set default estimates for new backlog items and tasks in the settings of a project. To go to the project settings, open the popup dialog for the project (in the "Projects" page), and click "Settings" in the upper right corner of the dialog.

The default estimates will be used when a new backlog item or task is created in that project. For example, if most of your backlog items have an estimate of 1 day, you can set this as the default estimate, to avoid having to specify it every time you create a new backlog item. 

The defaults do not affect existing backlog items or tasks, only new ones. Also, note that the defaults will be used for all persons creating backlog items or tasks in the project - it's not a personal setting just for you.