In your toolbar, go to Image > Re-size
Once you click "Re-size", the box below will pop up and you'll see a few options for re-sizing.
If you want to change by percentage, simply fill in the "bubble" for "By Percentage", type in the percentage you wish to re-size by and you'll see under "Pixel Size" how much smaller your image will be.
By Absolute Value:
Type in the width size you wish to have the width of you image in. So long as you have "Maintain Aspect Ratio", it will automatically re-size the height evenly.
If you un-check "Maintain Aspect Ratio", and you type in the width size you want, it will still automatically re-size your height, but it will not be symmetrical.