If you want to print it, you have to change the resolution. This will reduce the size when printed, assuming that you now have 72dpi and go to 150->300.
To change the size of the image on any monitor (I do suppose you're reffering to screen resolution), you have to make the bitmap smaller, meaning that PS will have to discard/kill [slick] /eliminate certain pixels, so that you lose information and quality.
Best way to do this is:
1/ do it in several steps
2/ always take random (though still smaller) numbers that have no connection (half, one third,...) to the original so PS cannot simply dicard, say one out of two pixels, but has to recalculate.
Because of the loss of information, your image will be a bit blurry and lack a tad of dynamic range. So Unsharp mask it a bit, and give it a gentle curves treatment to enhance the quality.
Remember that you van open a duplicate, so you can see and compare. Keep in mind that if PS shows something at less than 100%, you don't see all its pixels, so it will also have a blurry quality over it.