How to sharpen big size picture?

Hi, I have a question. Normally I use the unsharp mask in photoshop to sharpen the picture after I resized it. But if I do not resize my picture to a smaller size, the unsharp mask doesn't do much sharpening even though I tune it to highest sharpen effect. Anyone please recommend me a method to do this.
Thankyou.

Try increasing the pixel radius. Its the slider directly below the "Amount".