Constraint button > where is it in Image Size??

I am trying to resize a photograph while maintaining original file proportions.
but when I look at all of the forums it says to go to image size> check constrain proportions. I am currently running the most recent version of Photoshop on my desktop and my page does not contain this box?? So as you can well imagine I am not able to resize my image while maintaining proportions and it is driving me literally insane. Can someone please answer this for me if there is another way to do it? Please help!