You can define the topics to be discussed up front, based on data collected and affinity diagrams as you suggest. That would make it easier for participants to prepare. But you can also have the participants suggest topics at the beginning of the meeting, and do a dot voting and prioritization. Or use open space technology to have people organize themselves on topics that they consider important. Or turn it into a storytelling event where people share stories on what they experienced and learned.
There are several possible formats that you can use for larger retrospectives:
Open Space, where attendees bring up topics (preferable with data) and build their own agenda. Groups will mostly be small and as people are free to choose their engagement will be high
Having people first reflect alone, then with one other person, then in groups of 4, and then everyone involved based on 1-2-4-all from Liberating Structures. This exercise tends to engage people.
Doing an Artifacts Contest as described in book Project Retrospectives by Norm Kerth. Since this is more creative and people are doing stuff, it keeps them involved
Using the Bubble up exercise from the Agile Retrospective Resource Wiki. Relevant topics will bubble up, which increases the usefulness
A timeline retrospective where all attendees bring up highlights and lowlight will probably give a lot of data. To focus, look where there are many post-its, or where there's a bigger span of low and highlights
Make sure that topics are visible for all attendees. E.g. Use a beamer with live updates, large (A4 format) sticky notes, big markers, etc. This is crucial to keep people involved.
This is really challenging to conduct retrospective for a group of 50 members. The success of the retro is depends on the minimum number of people in a meeting. But i have an idea to conduct retrospective for a large group. Here is the details
Split the team members into a group of Developers, UI Designers, QA's
and Product owners.
Ask them to list the expectation and feedback of each teams.
Try to consolidate the points and discuss
This is more effective because it is a feedback from a group of experts.