There is an app called Image Size you can get from App Store that will do what you want. Be aware that lots of ads pop up so you may want to purchase it after you determine it does what you want and if you plan to use it in the future. For a one time use I would suffer thru the annoying ads!
The edit function of the Photos app that comes on your iPad may work close enough for your purposes. Select your photo, edit and use the crop function at the bottom (3rd from left)...play around with it a bit and maybe it will work for you.