Go to Image >> Image Size. In the Pixel Dimensions section, change the width and height to whatever you want. I'm going to change mine to 200x149. Make sure the properties at the bottom of the Image Size window match these properties:
(The Resample Image should be checked and it should also be set to 'Bicubic'):

(Important: Please make sure 'Constrain Proportions'is UNCHECKED! It'll make your image turn out yucky =P!)