You'll need to do a few things. You need to edit the admin/common/properties.php file and add your custom size to either the cropped or proportional array. By default the dissolve slideshow uses the cropped 782x481 size. So if you wanted to add 500x300 for example, you would add a line that looks like this to the array of your choice.
array('w' => 500, 'h' => 300, 'watermark' => true)
In your case it sounds like you want proportional images, so add it to that array. After that edit, you need to reprocess all the images that will use this size. You can do that by selecting your gallery, then go to settings and click the update watermark button.
Last step would be to edit the gallery itself - edit galleries/slideshow_dissolve/index.php and on line 14 change 782c to 500p. Basically 500 is the width you want and p is for proportional (c is for cropped).
As a first step, you can also just change the 782c to 782p to get proportional images loaded up instead. We use cropped images by default since the slideshow effects look better and more consistent using images that are all the same size.