How much notice will there be for the switch to 1.9? (and where will it be announced?)

This question is impossible to answer in a meaningful way.

1.9 has not be released yet. Even if Mojang announces a release day, there is no way to be sure the jar will be actually released on that day. This makes detailed advanced notice/detailed plans difficult. I will say that once Crafty has the jar in hand things will happen quickly. But watch the forums for announcements.

Is there a way for interested players to get more involved in planning for 1.9?

make a thread

Will there be more than 1 dragon egg obtainable in the world?

No idea.

Will there be access to an end portal from spawn like there is today?

Yes.

Will there be community input on villager trades for the new server? (and where?)

make a thread

What villager trades will be available?

No idea.

Will there be ranks (vip, vip+, etc)?

Yes.

Will there be periodic resets (i.e reset under 1k, reset end, etc.)

unknown, but no current plans for that

How large will the world be?

tentatively +/- 100,000

How long can we expect before another reset or major change?

no current plans for that

1.9 could be released tomorrow or 6 months from now. The exact plan for 1.9 is still not set in stone, but one thing has been decided: there are no plans to delete anyone's stuff.