1) Slightly smaller, but much denser. Think some of the expansive areas of Borderlands with more structures, and less wasted "desert" space. In the end it might end up being a bit bigger than BL2.
2) Yes, but in Destiny it's not really meant to segregate people based on supporting or not. Warlock is a regeneratively focused class but other skills for Titan and Hunter bolster other classes' abilities. If you come from Borderlands you should be familiar with some skills giving bonuses to the entire team or your teammates skills.
3) Customization is pretty expansive. There wasn't even gear shaders in the beta but it still seemed well thought out. And yes armor is reflected on your character model.