Because the 16" is so large, do I need to dowel and support between layers?

Do you mean tiers? If you are doing more than one tier then yes, you should always dowel. This adds support. It is better to be safe then sorry. You don't want your bottom tier squished by the top tier.

When you say "layers" do you mean that you're making a 16" two layer cake that has nothing stacked on top? If that's the case, you shouldn't need to put in dowels. They are only used if another cake is stacked on top, like when you're using 16", 12" and 8" stacked for a wedding cake. Does that help?