Valve has uploaded a character and texturing guide for the Dota2 community on the Steam Workshop page for Dota2.
These are essentially now your "art bibles" for modding Dota2. Pallets, silhouettes, geometry, UV's, and tons more are discussed to give you guidance to crafting some awesome pieces for the game. These are an awesome pair of documents. Even if you aren't interested in modding the game, I encourage you to download and take a look at them. They are great examples of the range of considerations that developers can/should/sometimes deal with when creating their own projects.
They are both pdf's so you can download them to your own machine and ponder them slowly off or online as you see fit. There is even some hints inside them that more testing tools are in the pipeline for us.