Hi Swapnil,
Here are some brief answers to your questions:
1. The greatest limitation of using an Access database is that they are not able to effectively handle multiple concurrent users. This makes use of Access for a project of any size/scope practically impossible. SQL and/or Oracle are able to manage this much more effectively
2. Storing specs in either project DB or an external DB is simply a setting in Project Administrator. If you open a project in Project Administrator, and go to System>Main you will see the options. Press the Help button while on that screen to see an explanation of the options. Other than a bit more security/stability, I don't see a great advantage in doing this for most projects.
3. I don't believe there really is a size limitation. You would have to see the limitations of each database provider, but I doubt you would run against any of these limits in a typical project.
4. SQL user permissions aren't really relevant to the AutoPLANT project, as it uses a single SQL user to give the project access to SQL. Security of the project, and by transition the database, is handled via AutoPLANT project administration through either the Project Administrator application or ProjectWise if you use it. AutoPLANT has limited security options without ProjectWise.
5. I haven't really used this in practice, but from what I remember it uses an ODBC connection that can achieve connection to an i-model for data retrieval and processing into your report. Once the i-model is generated and the i-model ODBC driver is installed you would simply go the the application (in your case Crystal Reports) and generate a report using a data connection to the i-model via the odbc connection. There is a video on the i-ware site demonstrating this process for excel, and crystal reports does have similar options.
6. AutoPLANT does not require any particular type of AutoCAD license. If your license is valid, AutoPLANT should work in my experience.