What is the average size of a residential bathroom?

Given that this question was posted under Entertainment and Arts, I'm going to assume that this office bathroom is to be used for the purposes of either Entertainment or Arts. That assumption leads to several different answers depending on more specific information.

1. Entertainment -

Situation A: You want to hold a large party in your office, but do not have enough floorspace to hold all your many guests.

Solution: A tastefully decorated bathroom can provide that extra bit of floorspace needed for when your party really gets jumping. The actual space you would need to accommodate above guests would of course depend on the number of guests, but if you go by the assumption that every person requires at least 3 square feet of personal space, more if you think your guests are very particular about personal space, should be safe. Remember, leaving the lid down can give that vital extra square foot or so.

Situation B: You want an office bathroom which is as entertaining as it is relieving.

Solution: This situation doesn't really need any special considerations for space, the standard bathroom size (i.e. whatever is in place now) should suffice. The trick is in how you use it. Flat panel monitors with movies playing take up very little space and keep busy workers entertained for hours.

Arts:

Situation C: Your office bathroom is functional, but not really very artsy.

Solution: Similar to Situation B above, this doesn't appear to need any extra space installed. If you do want it to appear bigger, try opposing mirrors to give that corridor effect. Or for a real kick, build the bathroom inside a spherical mirror. I don't know what that would look like inside, but I would love to find out.

Situation D: You want your office workers to feel as though they're contributing to the world of art by their vry presence in the bathroom.

Solution: Canvases in front of urinals, cameras in toilet bowls.