you know...maybe it's impossible because all the other users at the shared hosting server... or maybe it's related to the cpanel software used for the server management, updates, etc.
Try this:
Login to your Horde Imp
In the menu, click at the first item: "Horde"
Then, in the top-right of the page, click in "Options"
There go to the right side of the window, "Other Information", and click in:
QUOTE
Display Options
Set your startup application, color scheme, page refreshing, and other display options.
There you have:
[150] "Width of the Horde menu on the left (takes effect on next log-in)"
change the 150 by something more, log out and log in again.
It's magic!!! you will see your left menu wider.
good night, i'm going to sleep