Ok, I know this is a total abuse of rules, and by RAW, probably would not work, but bear with me.

1. So, if you consider the unarmed strike to be a strike by any part of your body, your body is effectively the entire weapon.

2. The kensai PrC can effectively enhance unarmed strikes with weapon abilities. (Though there are probably a few easier ways to get this without going through kensai, it's just the first I noticed)

3. Sizing, a +1 enhancement, can change the size of the weapon as a standard action....to any size.

Now, with these 3 things in mind, what is to stop you from making your unarmed strikes (AKA yourself) colossal/fine as a standard action?

(Just toying with random ideas here for the most part, and seeing what everyone thinks of this)