Google Image Search is now missing a neat and useful feature, to search by the exact image size. It is useful when you are looking for an image of an exact height and width.
So when it says, "search any size" - Google is lying. They mean only large, medium or icon sizes - any other size, well - you are out of luck.
The interesting thing is that this is not the first time this feature went missing, it also went missing in June 2011 and eventually came back.
Kousha from Google said in response in a Google Web Search Help thread:
Thanks for checking in about this. We're aware that the exact image search no longer appears in the new layout. We're currently working on implementing new features like the one you describe, and will check back in with any updates.
Forum discussion at Google Web Search Help.