Take my word for it, as I have done alot of Photoshop work.  AND I consider myself
a good manipulator of photographs.

One of the first mistakes a new Photoshop user makes it retouching images is: trying to focus
a soft, or slightly out of focus image. It CAN'T be done. Stop trying.

Sharpening an images only increases the contrast between opposing pixels. It will not
in any way even appear to closely focus your image. Stop trying.

My word of honor on this tip....I promise

It is not impossible in my opinion. I am currently working on some pics. It depends how much detail you image has. For example I would never try to fix an human face with hairs.
But there is no fast way. It is a lot of work. Depending on the image it can take hours.  I tried it on one image wich I interpolated about to 4times the original size. Then I started to sharpen the edges with various tools, which worked well. After about two hours the Computer crashed and my work was totally gone. However, I experienced it was makeable.  (I only worked on it so long, because I thought it would bring in good money..and was curious)
The next image I worked on was a moon at night which I took a photo of with a D60 @300mm. Still the moon was much to small. I cropped the moon, and interpolated it 8-10 times the original size. I sharpened the shape of the moon (Of course not with the sharpening tool!) and did some fixing of the moons surface and the sky, then downsized it to approximately 8Mp and uploaded it.
Lets see if it will get accepted :-)
DT might be too picky, because its only a moon, and they have many moons. (At DT you never know..) But SS probably would accept it. I will upload it there in the next days too (It is completely noisefree .

In addtition I photoshopped a comet out of a moon, in the beginning it was an accident, which then inspired me to do the comet. So the moon-comet is also like 5-6 times the original size and I am quite confident  they wont reject that image because of lack of focus.  

So for a general detailed image it is much too timeconsuming. It might take several hours up to days if you really want to sharpen a detailed image. (Which is usually not worth the effort.)

I will show you leafs method, since it is a lot faster
Here it is.. If that is not focused what then is focused

The smaller the final size, naturally the smaller the print you will be able to generate with very good quality.

I have used the downsizing technique on occasions.  It also helps to reduce chromatic aberration, if it is not too strong (if it is, it will be still noticeable at smaller sizes).

Regards,
Adelaide

Gee, that's not good. So in fact you can't make an out of focus image in focus.
You can just reduce it's size and make it appear to be in focus, but at a loss of
reducing it's print size.

Right?