Sure, I can provide a bit of extra information!
I'm currently creating the engine - it's a tile engine that can generate and run maps up to 268 million x 268 million blocks in size (each block is 16x16 pixels).
(Terraria's largest map is 8400 x 2400 tiles, compared)
The game can be played as both singleplayer or multiplayer (so people can join together to explore and create together).
I'm still designing and adding details and ideas, but I have a rough idea of what it will look like. I just thought it'd be nice to get an artist on board right from the start.
So, as to the setting of the game, it's an alien planet, similar to earth, but a bit different, too.
The planet has some alien technology on it.
Environments will range from the usual dirt and grass, to stone, water, lava, mountains, caves, etc. (with some alien environments too)
As to the amount of graphics I'd need...
- terrain tilesets, probably about 6 distinct ones, with the rest being modifications of the others.
- trees, crystals, and other objects
- player graphic - a person in a power suit / mech suit / power armor - animated for running left/right, jumping, and looking towards where the mouse is pointing. Male and female versions would be required, but can be very similar.
- creatures (animated for moving and attacking)
- robots (animated for moving and attacking)
- technology items (player's base items, like turrets, generators, elevators, etc.)
There is no colour limit - I'll be using 32-bit pngs, with 8-bit alpha transparency.
There are also some ideas and details that I'd like to leave hidden at this stage

But I hope that helps!
-EDIT-
Removed some information about the game, as I don't want to reveal too much
