Look for PPI (Pixels/inch)
Go to Image>resize>image size. You can resample, be sure that constrain proportions is checked.
However, as you as increase ppi, the dimensions (H&W) will decrease.
For printing, it is desirable to have the ppi in the 240-300 range. I have printed satisfactorily below this range, esp. larger pictures where the observer's eye will be at a distance.