All we need to clarify, are the dimensions. I think it should be standard size, so we all have similar outcomes.
I suggest using 400x600px. Any objections?
(Another option is 500x750px)

(Note: I'm remaining a 2:3 ratio, which is close to poster size)

Edit: Oh yeah, and how long do you guys want to do this? 1 week? 2 weeks? 10!?!