i do scans @ 300dpi (i could do even 1200, but thats a format for elite huge books, so it doesnt make sense to me)
then i move to the format asked by the site... here in topastic should be nomore than 940px. on deviantart, usually, i dont go to the original full size (i upload a resized image, maybe 75% from original, anyway not big enough to go in the printing process @ full quality)
btw, what i learned about this kind of resize it's about tones (something that could be write as a "the details" chapter). this mean, when u downscale, you loose details... so, tones (that kind of dot tones used in the 70 and so on) become just a flat scale of gay : \
but also, if u do some small detail and u do resize, say goodbye to them.
and in your case could be the texture... so take it in mind when u work.