A poem with grandly conceived and executed stanzas, such as one of Keats's odes, should be like an enfilade of rooms in a palace: one proceeds, with eager anticipation, from room to room.

At four lines, with the quatrain, we reach the basic stanza form familiar from a whole range of English poetic practice. This is the length of the ballad stanza, the verse of a hymn, and innumerable other kinds of verse.

I don't see that a single line can constitute a stanza, although it can constitute a whole poem.

James Fenton Quotes