If you're sending your LO to print (by uploading or whatever), you can specify the output size with the vendor.
If you want to permanently change the output size in your program, you need to go to whatever menu item in your application covers the image and canvas size. In Photoshop, that's Image--Image Size. You can change one dimension and keep the proportionality and resolution the same. Just be sure you keep the options checked for "Scale Styles" "Constrain Proportions" and "Resample Image". If you're using PE, I think it's pretty much the same. Other programs will have their own names, but you should be able to find the correct settings.
If you're having trouble, post again, and specify your program so we can help.