At the:
\concrete\core\controllers\blocks\guestbook.php
add this lines of code to action_form_save_entry() method (line 117)

change 10 to whatever length you want.

Thanks - I'll test this out.

I'm using 5.6.1.2
Ok basic question: If I'm making a change to the Guestbook.php where do I put My customized version? Root\block\??... for the \concrete\core\controllers\blocks\guestbook.php

1- Copy \concrete\blocks\guestbook\*.* to \blocks\guestbook\*.*
2- open controller.php at the newly copied folder. you see 2 empty extended class.
add action_form_save_entry() method (from file: \concrete\core\controllers\blocks\guestbook.php) inside first class.
now you can modify this method instead of one in the core folder.

Sorry don't know what you mean "Inside the controller" your 2 point.

The change is to the "Guessbook.php" I'm curious ..what does the guestbook/controller.php have to do with it.
Shouldn't guestbook.php just go to root\blocks\guestbook.php?
I wish to get it pointed correctly.

for understanding how the 'overrides' at c5 works, check the documentation.
but in brief for a core blocks like guestbook we have this hierarchy:

\concrete\core\controllers\blocks\guestbook.php
->Inherit:
\concrete\blocks\guestbook\controller.php

now that you want to override a new class you should override from second one, so copy \concrete\blocks\guestbook\controller.php to \blocks\guestbook\controller.php

but the point is you want to modify a method from first one, so copy mentioned method from first class.

Thanks - I'll read up to get a better understanding of overrides.