The best I can do easily is to make a Web.config setting:
<add key="FolderGalleryPreviewWidth" value="500" />
Previously the preview size for folder gallery was hard coded at 700 which is rather wide for most of our skins. Its not something I can easily make a module setting for (like the other gallery) because of the way the folder gallery works, its an http handler and really has no idea internally that it is a also paert of a module so it can't use module settings, so the best easy thing I could do is make a Web.config setting.
I just implemented it on my local copy and it works, so it will be in the next release of mojoPortal.