How and when in SCRUM do you establish the team and its size?

We develop products and want to use SCRUM for development. We start with business case, high-level business analysis and technical outline that all contribute to and form the backlog items. So after a month or so, we have the high-level features captured in the product backlog, keeping in mind it might change. So now we should decide on the team...how should I do that? How to tell whether 2 or 6 are needed, what is the best practice?

Solution

Usually SCRUM goes like this:

Depending upon the backlog and complexity of tasks a development team of 4-8 individuals is created which typically includes designer, architect, developer, tester and a scrum master (tasks like: analysis, design, development, review, testing & technical documentation).
You can decide on sprint cycle's length including a separate planning period
In planning period you assign tasks to individuals and the effort estimates based on availability of resources and time
After planning, you track the progress of tasks and update your backlog list accordingly

As SCRUM is supposed to be self organized, there are times when you might need some interaction from project managers or domain experts.

After each sprint cycle, ideally there should be some dedicated time for sprint analysis which can give inputs to next planning phase.