Q:

Best Firebird blob size page size relation

I have a small Firebird 2.5 database with a blob field called "note" declared as this: 
BLOB SUB_TYPE 1 SEGMENT SIZE 80 CHARACTER SET UTF8

The data base page size is:   
16.384 (That I'm suspecting is too high)

I have ran this select in order to discover the average size of the blob fields available:
select avg(octet_length(items.note)) from items

and got this information:
2.671

As a beginner, I would like to know the better segment size for this blob field and the best database page size in your opinion (I know that this depends of other information, but I still don't know how to figure it out).

A:

Blobs in Firebird are stored in separate pages of your database. The exact storage format depends on the size of your blob. As described in Blob Internal Storage:

Blobs are created as part of a data row, but because a blob could be
  of unlimited length, what is actually stored with the data row is a
  blobid, the data for the blob is stored separately on special blob
  pages elsewhere in the database.
[..]
A blob page stores data for a blob. For large blobs, the blob page
  could actually be a blob pointer page, i.e. be used to store pointers
  to other blob pages. For each blob that is created a blob record is
  defined, the blob record contains the location of the blob data, and
  some information about the blobs contents that will be useful to the
  engine when it is trying to retrieve the blob. The blob data could be
  stored in three slightly different ways. The storage mechanism is
  determined by the size of the blob, and is identified by its level
  number (0, 1 or 2). All blobs are initially created as level 0, but
  will be transformed to level 1 or 2 as their size increases.
A level 0 blob is a blob that can fit on the same page as the blob
  header record, for a data page of 4096 bytes, this would be a blob of
  approximately 4052 bytes (Page overhead - slot - blob record header).

In other words, if your average size of blobs is 2671 bytes (and most larger ones are still smaller than +/- 4000 bytes), then likely a page size of 4096 is optimal as it will reduce wasted space from on average 16340 - 2671 = 13669 bytes to 4052 - 2671 = 1381 bytes.
However for performance itself this likely hardly going to matter, and smaller page sizes have other effects that you will need to take into account. For example a smaller page size will also reduce the maximum size of a CHAR/VARCHAR index key, indexes might become deeper (more levels), and less records fit in a single page (or wider records become split over multiple pages).
Without measuring and testing it is hard to say if using 4096 for the page size is the right size for your database.
As to segment sizes: it is a historic artifact that is best ignored (and left off). Sometimes applications or drivers incorrectly assume that blobs need to be written or read in the specified segment size. In those rare cases specifying a larger segment size might improve performance. If you leave it off, Firebird will default to a value of 80.
From Binary Data Types:

Segment Size: Specifying the BLOB segment is throwback to times past,
  when applications for working with BLOB data were written in C
  (Embedded SQL) with the help of the gpre pre-compiler. Nowadays, it is
  effectively irrelevant. The segment size for BLOB data is determined
  by the client side and is usually larger than the data page size, in
  any case.

