What shape is the painting?
Paintings usually come in three shapes: Portrait, Landscape and Other.
Portrait - taller than it is wide, and the rough shape of a person. Ideal for - well - portraits. Artists also choose it when they want to emphasise the height of a subject.
Symphony in White, No. 1: The White Girl
1862
Whistler
Four Time of the Day: Noon
1860?
Corot
Landscape - the clue is in the name. Not only used for areas of countryside and so on, this shape lends itself to paintings which tell a story.
Winter Timber
2009
Hockney
Whence came we? What are we? Whither go we?
1897
Gauguin
Other. Very occasionally an artist will choose a different shape (and sometimes this is imposed on them when a painting has to fit in with, say, architecture). Circular paintings are often called tondos.