Actually there are no rules in steemit about sizes of photos.. But with my personal experience and preference, the image size shouldn't too large. There are some images that cover the whole page. It is not pleasant. Those images should be resized.
look:

whole screen is covered. Readers need to scroll multiple times to pass the image.

Use your creativity to minimize the size. Not too large nor too small. (for vertical images) The following one is nicely displayed.

_{Original post}

The second point of mine is, Horizontal images add more neatness than verticle images. It is nice using full space left to right than bottom to top.

@anroja is some one perfectly use photos in his posts. If he see this he can also give an advice for us : check this from anroja. You will love it