Resizing Photos

Again there are two options:

To change the aspect ratio or to crop elects out of the image and resize it that way, you can use the crop tool in Photos or Preview. You'll have more options in Photos.

To scale the image - that is, reduce the size without removing any elements of it - you can do this while exporting from Photos. Check out the options under Size:

Custom will allow you to specify a particular dimension in pixels.

Several ways using either Photos or Preview, but before I go further, what do you mean by 'resize'? Change the file size measured in MB or KB? Or change the dimensions of the photograph (length by breadth) ?