To identify the number of meetings with the most participants and/or longest duration in CPM Analytics dashboards, navigate to the Dashboard and add a new panel as shown below:
When you click on the bar where it shows >12 it will navigate to the Meetings tab and show you meetings that had more than 12 participants in it.
To see the meeting with longest duration you can sort on the Meeting minutes column.
Tip: You can use the date range picker in the top right of any tab in CPM Analytics to change the dates you want to analyze. You can click on the button at the top right corner of your page to schedule a one time report or recurring report.