Making all images on a post the same width and height proportionally

Hello-

I’m new to wordpress so this is probably a simple question, except I can find a forum that answers it. I want to figure out how to make all the images the same width and height on a post and keep each image proportional. I mean that since some of the images are rectangles and some are squares, I want all of the image to be the same width and height without removing the edges of some of the images.

This is how I go about uploading and adding images. Please let me know what I am doing wrong. Surprisingly, I could not find a simple to understand answer online.

-I’ll find a image online and save it as a jpeg.
-If the file size in MB or KB’s is greater than 150KB, I will optimize the jpeg and make it no more than 100KB.
-I then upload the media to my library.
-I then go to the post/page that I want to add the media to and I usually either select medium or thumbnail.
-I don’t do any other editing to it.

If I have a post/page where I want every image to be exactly the same width and height, and I do not want to have the image have the sides removed (for example if its a rectangle), I cannot figure out how to do this for just that post/page. I’ve seen examples online of people saying to enter code except I don’t want to change every image size on the website. Thanks.