Hello,
Using Zenphoto 1.6.5.
The Gallery description size (Options section ->Gallery) of the admin page is not large enough for me :

Each time I upgrade, I modify the setting to get :

Maybe for a future release !

We can consider to raise the height. But since you are using tinymCE you can switch to literally full screen, too.

There is a new setting in tinyMCE plugin already just for that - Text editor height (added in 1.6.5)

Won't solve the width in the options (it seems to be hardcoded at 350px) - but you can adjust height of all text editor fields, that use tinyMCE