Well for the right placement you have to import the doom model and use bones from that model and set the right bone as parent to your model. Which one should be mentioned on Valves requirements for the hero. For scale I'd just download his original sword and compare it in 3d editor.
Isn't one of the defining characteristics of Baba Yaga her house on a chicken leg that hops around? Maybe instead of a cauldron, she could be riding a platform/hut/chicken leg, haha :) I like the top right color scheme - or the bottom right if the first is too muted. Nice idea!
Your axe has an interesting shape, however the diffuse kills it right now. It is fairly obvious to me that you used a textured brush all over it but you need to work on clearly defining the major shapes. Right now I can't tell if your axe is made of stone or metal.