The display on the Kindle Fire Hd is 7 inch , but the machine is bigger than seven inch.
As far as being able to look at Youtube and other none amazon apps.......the word is to download dolphin browser and use this along with VLC for youtube.......now I can't enlighten you on if these work, but feedback from the amazon site says they do.
I found this too...maybe it will help you Rob(I know you are much more technically adventurous than me)
This guide helped me to side load How To Sideload Android Apps on the Amazon Kindle Fire in 3 Steps - Tested