The Scrum Guide
The Sprint Planning meeting has two topics, not two separate events.
Sprint Planning is time-boxed to a maximum of eight hours for a one-month Sprint. For shorter Sprints, the event is usually shorter.
The event is not required to be shorter.
Work planned for the first days of the Sprint by the Development Team is decomposed by the end of this meeting, often to units of one day or less.
This emergence occurs as the Development Team works through the plan and learns more about the work needed to achieve the Sprint Goal.
The effort of the Sprint should not be overly planned: the work emerges.
any one item can reasonably be "Done" within the Sprint time-box. Product Backlog items that can be "Done" by the Development Team within one Sprint are deemed "Ready" for selection in a Sprint Planning.
Be certain the the item is sufficiently understood by the Development Team prior to Sprint Planning. (See my answer to your other question.)
Vertical slicing(1, 2, 3) is an important technique for doing just enough of a feature to provide value. When the scope of the item is smaller it is more easily understood, decomposed, implemented, tested, and (ultimately) delivered.
Tasking is often a sign of an opportunity to improve collaboration. Individual pieces of work being determined and being addressed by individuals (especially in a development versus testing versus documentation manner) invites bugs and technical debt among other issues. Try pair programming in order to provide code review as work occurs and enable shared knowledge.