All the thumbnails are the same size. That is the ones in the index page, the ones in the svManager images page and the ones inside SimpleViewer galleries. So the easiest way to get larger thumbnails in the index page is to increase the size of all the thumbnails. You won't see any change inside SimpleViewer because it resizes to it's own default of 65px. You won't see any change in the svManager images page because the browser will display at 65px according to the attributes of the img tag.
To do this, change the following settings in the svmanager/includes/constants.php file:
// Actual size of thumbnail image in pixels
// Width attribute for <img> tag in galleries index
// Height attribute for <img> tag in galleries index
Change the 65 to your new size.
Obviously this is a bit wasteful on disk space and bandwidth since you are creating larger thumbnails and reducing them again but the real disk space and bandwidth use is in the images themselves. In theory there is also a reduction of the quality of thumbnails in SimpleViewer but I just ran a test and I can't really see any difference.
SimpleViewer Support Team.