Need to make portrait photo a landscape photo by adding width to canvas, and reduce size of photo

I have a photo that is 3024 x 4032 pixels, 72 dpi; so it is 3 x 4 aspect ratio. I need to add dimension to the sides to make it a landscape photo, 4 x 3 aspect ratio. I don't want to crop any of the content off the top or bottom of the photo. I also need to make it a smaller file size, how small exactly I don't know; I may have to use trial and error to figure that out. (I need to post photos to an online listing site.) 

Does it matter if I make the photo smaller first, then add horizontal area to the canvas, or vice versa? How would I figure how much area to add to the horizontal canvas? I know that the features to use for this are "Image Size" and "Canvas", I just don't know how to go about using them to achieve my goal.

I will greatly appreciate any help!