When posting work that is yours, note it with an [OC] for original content.
When posting work that is NOT yours, post the original artist. If artist is unknown, try to do a reverse image search (such as google image search or TinEye.com). If the artist cannot be discovered, please note this with [Unknown Artist]. Posting of other's work without attribution will lead to the post being removed. Repeated offenses will lead to a ban.
Size notation is only important on photos of physical pieces as this can provide some context for how we are supposed to view the piece.
Where possible, it is also great to note what materials were used in the piece. For example: Digital, graphite on paper, oil on canvas, and so on. This is not required but it is a very nice thing to do.
It drives me insane that it goes from most done to least done, instead of the other way. Still cool, but I can't get over it.