Well in this post you will find out how you can download the original size of a Youtube thumbnail that you want.
1. Click on the video you want thumbnail from.
2. Press and hold "Ctrl" and then click on the letter "U" (Ctrl+U).
3. Press and hold "Ctrl" and then click on the letter "F" (Ctrl+F).
4. Copy "thumbnailurl" and paste in the small search box (usually at the bottom of the page).
5. Click on the URL next to thumbnailurl.
If you are a Youtuber then you migh ask what's the best custom thumbnail size?
Well, what I have found the most perfect size was 791x483 (Width=791pixels and Height 483pixels).