Whenever you create a new post and decide to add a picture or video, you will see in the editor a way for you to add content in an horizontal view as well as a vertical view. The particularity of the system is that you can add both types of views at the same time.
You simply need to choose what part of the vertical picture should appear or be cropped when viewed in a horizontal/compact mode. When someone click on that picture or video, they will then be able to see the content in a full/vertical mode. You don't need to create two types of content for those 2 views to exist. You do it directly in a single shot in the editor as you create your post.
Regarding the size of the post, try to have a ration which is:
- 2600px height x 1500px width or similarly
- 1300px height x 750px width
As long as it is the same ratio you can use any dimensions. The larger the number of pixels, the better the quality of the image. Try to avoid uploading images that are too heavy.