In regards to size I believe 10" to be around the minimum but 12" - 16" is the sweet spot for most locations but this depends on your local seeing conditions of course.
When it comes to design I have seen good images from these designs:
SCT and the variants (Edge etc)
All designs have their quirks you just have to figure out what they are and how to overcome them to get the most out of your telescope.
I have to say the best telescope is the one you will use the most!
Its worth mentioning the focal length of the telescope should be larger enough so the least amount of glass is used in the system. For example having the focal length large enough so you don't have to use a barlow!
Attached is an image from a 11" edge telescope under under good conditions. Planet killers come in all shapes and sizes as long as you have the skills.
Very nice image!