How do I determine my sizes for a world?

[verdilak]

I am currently in the process of creating a earth-sized world and I am needing to know what should my image size be? Looking at most world maps, they are more rectangular than square, so i am thinking that square isnt the way to go. Also, it seems like there should at least be some sort of polar/ice cap section on each of the poles.

The world will be rather water-filled, lots of islands with 1-3 larger landmasses.

So, I was thinking that i read somewhere around here that to make the earth, you would need something like a map size of around 25000 x 25000 pixels, but that seems rather too ridgedly squarish compared to other world maps.