You'll need to re-process the image yes. However, a quick way to do it is to update the image watermark. This will re-size all the images without you actually having to re-upload them all.
All the front end designs are in the galleries folder. If you remove a gallery from there, it won't show up in the admin. If you add a new one, it will show up in the admin. Each gallery has a unique admin/design.xml which controls the name and description that shows up in the admin. For each css style (i.e. dark, light), there is a dark.png and light.png that are the thumbnails in the admin.
The properties.php file contains all the sizes, so just like adding the lines of code will create the new sizes, removing the lines you don't want will not create those sizes. I would recommend keeping all the default sizes though since all the built in galleries and admin need all those sizes to work properly.