For a blue skinned model I would still make the base white. But then I usually always make my bases white. Only exception is if you want to use a specific texture and have the final character look black or asian. In those cases you could tint the base. The size of textures depends on several things. I can see you're making textures for Posette. I don't remember the "normal" size of her maps but for Victoria I would say 3000x3000 is a high rez map. I guess the same would apply to Posette.

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You just can't put the words "Poserites" and "happy" in the same sentence - didn't you know that? LaurieA
  Using Poser since 2002. Currently at Version 11.1 - Win 10.