We say that this poem is written in free verse, meaning there's no metrical pattern here. No pattern of stressed and unstressed syllables – though there are some cool metrical moments (check out the way that "but I laugh / and eat well / and grow strong" [lines 5-7] repeats that lilting da-da-DUM rhythm three times in a row). One thing that really sticks out about the poem, though, is the fact that all the lines are really, really short. In some cases, they're only one word long! What's up with that?
One of the problems with tiny poems is that they can go by very quickly, leaving the reader without a good, full appreciation for what just happened. One way to fix this is to shorten the lines.
Whoa, hold up, you say. Wouldn't shorter lines go by faster? You know, because short is a lot like fast?
Not so fast. (Hee hee, we kill ourselves here.) One of the very cool things about poetry is the way in which it can control, even more precisely in some cases than prose, the speed at which the reader reads. It takes a while – speaking in reading terms – to move your eyeballs from one line to the next in a poem. That pause is much longer than the pause between, say, a period and the next word of a sentence, usually. So, in the case of "I, Too, Sing America," all those tiny lines serve to really draaaaaaawww ooooooouuut the time you spend reading the poem. Which is a good thing, considering that there are a paltry six (very short) sentences in the whole piece.
So, in the great "free verse isn't free" tradition (like the old "freedom isn't free" cliché, but for poetry), this seemingly non-formal poem is actually carefully constructed, giving the reader plenty of time to read and absorb each individual thought in the poem. There's no rush here, and that's a good thing.