to change the width, go to the Choose design tab and click the settings link for the galery you are using.
in the images section, you can set the width of the thumbnails, large, and supper size images.
for the 500 server error, This usually happens as a result of file permissions issues.
to fix the problem, make sure that all files and folders under the /galleries folder are set to 755 permissions, you can do this in most FTP clients like filezilla by right clicking on the webassist folder in the file panel and selecting the Set File Permissions option, setting the 755 permission level then selecting the option to apply this to "Recurs into subdirectories" -> "Apply to all files and directories"
Also, edit the admin_gallery/common/properties.php file, change the following code:
$GLOBALS['permissions'] = array(
'dir' => 0777,
'file' => 0666
);
to:
$GLOBALS['permissions'] = array(
'dir' => 0755,
'file' => 0755
);