Using the Insert Gallery Window, you can apply custom gallery settings on a per gallery instance. To set the default-global setting, these changes would be made on the gallery settings page, under Gallery >> Gallery Settings (*important to note that changing these settings won't update any existing galleries previously added to a post or page):
Basic Gallery Settings Overview
Set how each gallery within this album style displays (i.e. thumbnail, slideshow, imagebrowser...)
Set thumbnail size, aspect ratio and thumbnail quality as well as other thumbnail gallery settings here that will be set for each gallery as the default.
Set slideshow size, transition effect, and other options.
Set the default template for imagebrowser.
Set the default style settings for single images inserted into a page or post from a previously created NextGEN Gallery.
Set the default gallery style to display when tag link is selected in tagcloud.