1. What is the content growth over a period of time. So we should consider how much content is added per day, Week and per month. This growth rate will help to calculate storage requirements and do database sizing calculations.
2. We should find how much of this content will be available while launching the sites. It may happen that all content is not required to be migrated. This will help to determine initial sizes of content databases, temp db, search database.
3. The next thing to consider is how much data needs to be active and available and how much can go to the archived state. This helps to find the number of content db required.
4. What are the average and maximum sizes for each distinct type of document content? This helps to design the storage in SQL Content DB v/s File System.
5. What is the time available for the SQL Backup's and restoration. Helps to find the size of Content DB's.
These are some of the considerations needs to be taken care while designing the large content DB's in SharePoint.