Changes have been made in Git (master):

commit f66d2d1caf346da34c30619a33edd6b9e1c75a10
Author: Michael J Rubinsky <mrubinsk@horde.org>
Date:   Thu May 12 17:08:54 2016 -0400

     Bug: 14353 Make calendar descriptions a text field.

  .../27_kronolith_upgrade_description_totext.php    | 41 
++++++++++++++++++++++
  1 file changed, 41 insertions(+)

http://github.com/horde/horde/commit/f66d2d1caf346da34c30619a33edd6b9e1c75a10

Depends on Bug: 14357

This won't likely happen until Kronolith 5 / Horde 6. However, you can 
change the definition of the column on your own directly in MySQL to 
type 'text'.

Depending on what share backend you are using (SQL or SQL-NG), you 
would do something like this:

ALTER TABLE kronolith_sharesng MODIFY COLUMN attribute_desc text;

or
ALTER TABLE kronolith_shares MODIFY COLUMN attribute_desc text;

Yes, we are using a MySQL database

I've checked again this issue.

I've seen that Agenda Description shows more than 256 characters while 
user don't logout, perhaps full description remains in a internal 
variable without the 256 char restriction.

But after you do logout, you will see only 256 characters on next 
login, so it's limited in SQL database as you has explained.

This weird effect provokes that users complain because they doesn't 
notice that  saved description is truncated until next login.
We would appreciate that you could increase the 256 characters limit 
on description tab in next versions.

I will assume you are using SQL for the share system?
This is because the database schema limits the description field to 256.
Not sure how this is happening, it should be truncated on the original 
save as well.

We will need to change the schema for the description field, but this 
will probably have to wait until Horde 6 since there are already new 
migrations added for Kronolith unless another developer has an idea on 
how to add a ".5" migration...

UserA creates CalendarA, its  Description tab has more than 256 
characters, then CalendarA is shared with several other users, i.e. 
userB.

CalendarA retains the complete Description when it's saved.

Then UserB has CalendarA as a shared calendar, but he only can see 256 
caracters as much in CalendarA Description tab.

Some Calendars have relevant descriptions and the truncated 
information are causing issues to our users.