What's the limit on the Firebird database size?

The database file size limits are:

32 TB for Firebird 2.x
64 TB for Firebird 3.x

The database size is limited by the number of pages, which is stored as a 32-bit integer. Each page can hold from 1kB to 8kB of data. Traditionally, using 4096 byte pages was recommended because many operating systems used to write data to disk in 4kB batches. If you need to have a really big database file, make sure you use the 8kB page size.

You also need to consider filesystem limits. If your filesystem doesn't support big files, you can use ALTER DATABASE to split the Firebird database across multiple files. For existing databases, you can backup and then restore to multiple files.

Here are some facts:

Windows:
FAT12 has a 32MB file size limit
FAT16 has a 2GB file size limit (Windows NT 3 or newer)
FAT32 has a 2GB file size limit
FAT32 has a 4GB file size limit (Windows NT5 / 2000 and newer)
NTFS has a 4GB file size limit (Windows NT)
NTFS has a 16EiB file size limit (Windows NT5 / 2000 and newer)

Linux:
EXT2/EXT3 (with 4kB blocksize) has a 2TB file size limit
EXT2/EXT3 (with 8kB blocksize) has a 64TB file size limit
ReiserFS 3.6 has a 1EiB file size limit
XFS has a 8EiB file size limit
JFS has a 8EiB file size limit

If you wonder what EiB is, it's a measue like MB, GB, TB, etc. Detailed explanation can be found here:

http://physics.nist.gov/cuu/Units/binary.html