MySQL what is the maximum size of a database?

According to the MySQL Manual:

E.10.3. Limits on Table Size
The effective maximum table size for MySQL databases is usually determined by operating system constraints on file sizes, not by MySQL internal limits. The following table lists some examples of operating system file-size limits. This is only a rough guide and is not intended to be definitive. For the most up-to-date information, be sure to check the documentation specific to your operating system.

Operating System                     File-size Limit
Win32 w/ FAT/FAT32                   2GB/4GB
Win32 w/ NTFS                        2TB (possibly larger)
Linux 2.2-Intel 32-bit               2GB (LFS: 4GB)
Linux 2.4+  (using ext3 file system) 4TB
Solaris 9/10                         16TB
MacOS X w/ HFS+                      2TB

Windows users, please note that FAT and VFAT (FAT32) are not considered suitable for production use with MySQL. Use NTFS instead.
On Linux 2.2, you can get MyISAM tables larger than 2GB in size by using the Large File Support (LFS) patch for the ext2 file system. Most current Linux distributions are based on kernel 2.4 or higher and include all the required LFS patches. On Linux 2.4, patches also exist for ReiserFS to get support for big files (up to 2TB). With JFS and XFS, petabyte and larger files are possible on Linux.

As for the other part of your question, a few thoughts:

1. It's a broad, complex, multi-factorial question. Consider narrowing the scope of the question to MySQL and one other RDBMS (eg. SQL Server) and probably even one particular feature.
2. Google is your friend.
3. Vendors tend to publish their own biased comparisons. Take vendor numbers with a grain of salt.