Size of preview window

How can I set the size of the preview window?

The preview window's size is always 1/3 of screenwidth (other 2/3 are the dual listers). For reading docs it's too small. If I change it DO only remembers until next full restart.

[How to save the positions of the Tree, Find and Viewer panes)

Sorry, didn't found that. I've searched for "preview". :blush: