4 times the original is what we have been using for duke. Use whatever you want though.
Some advice:
No such thing as too big. It can be shrunk easily enough.
Stick to integer size multiples. Preferably powers of two, though as long as it works it doesn't really matter.
Stick to the original ratio!
Don't use the original texture. Start from a blank. Now, if you happen to be looking really closely at the original while you are doing it...
As far as ripped Anime goes, if you can find what they ripped, go for it. I think just putting in an ad type poster or a cover isn't that big a deal. Probably get chewed on for saying this, but whatever.
Though if you know anyone who can draw Anime style that would be awesome. I know a guy whos damn good, but I don't know if he would be interested or have the time. Finals are a week a way here (though not for me
[co-op])