> Oops, I forgot to ask, does Firebird define a constant for the max
> size of a blob segment?

No, Firebird have no such constant. But, if you look into API, you'll found
that both isc_get_segment and isc_put_segment used unsigned short (2-bytes
len) as segment size. Therefore maximum chunk which can be reads or written
via one API call is 65535 bytes. Despite of segment size in blob declaration.

Regards,
Vlad