I want to stay 27.5" it feels more playful to me.
Full suspension for sure for me.
That said: Agility is more important to me than monster trucking over boulders.
My current bike is a large. At 5'10" do I want a medium or medium/large?
I love my Diamondback release 2, short chain stay, playful geometry. A new one will set you back just over budget though. Didn't find any used in my search.