What are the dimensions of a short poem?

Is the short poem defined by:

number of lines
length of line/number of words
ratio of silence to sound
subject matter
the number of seconds it takes to read at an open mic (26 seconds)?

Can a poem be one word? Probably

Footnote to 1/2/10 on beets.
Best served at room temperature. Roast more than you need – they keep well, and your friends will love you.

Beets have no concept of boundaries, no concept of what a line is. Pomegranates, too.