It really depends on what you're uploading it for (i.e. are you asking what will fit within the Guild's attachment size limit?). If you're looking to keep file size manageable for loading times/upload limits etc, then it depends upon how many colors you have.
If file size is no object, and you just want something viewable, you can make it any size. What matters is the resolution (most monitors are between 72 and 150 pixels per inch).
Hope that helps; if not, try to narrow the question down a bit