FWIW, as of Horde 5.2 we now default to a maxsize of 64kb for ANY preference.

To solve my problem i have to change locally the mysql database schema 
and/or add some php code to throw error exceptions if a given input 
size is exceeded. I assume this breaks the horde online update function.

Nope - we aren't limiting to 64k.  By default, we use the 'text' SQL 
type.  That's unlimited on, for example, postgresql.   
(http://www.postgresql.org/docs/9.3/static/datatype-character.html)

If you use a DB that imposes a limit that you feel is too small, 
change it to a more appropriate data type locally.  We provide default 
SQL schemas that should work with most systems, but due to the 
variability in DB implementations we can't guarantee the schemas will 
fit everybodys needs perfectly.

To start with imp/pgp functions i used the autoresponder 
adele@gnupp.de. Due to lots of signatures the public key file 
(0x92AB3FF7) has about 96k size.

Two problems are detected with this input:
(1) Importing directly from a response mail with attached keyfile 
causes the import of another public keyfile from the same sender 
(0x346D74D5) but not from the mail attached file.
(2) An Import via copy/paste or file import stores a truncated keyfile.

Both variants import successfully ! Variant1 encrypts but fails on 
reception, Variant2 fails on encryption.

An obvious reason for at least Variant2 is the limited size (type 
'text', 64k) of object_pgppublickey.