To answer your question, take the poem and put a small mark between each syllable in each line of the poem and add up the number of syllables (I'm simply using the / out of convenience here):
Hold / fast / to / dreams = 4
For / if / dreams / die = 4
Life / is / a / bro / ken- / winged / bird = 7?
That / can / not / fly. = 4
Hold fast to dreams = 4
For when dreams go = 4
Life is a barren field = again, 7?
Frozen with snow. = 4
The answer, then, will depend on what lines in the poem you are talking about. Hughes' poetry tends to follow the natural pattern of spoken English, which is called "iambic" and is characterized by alternating unstressed and stressed syllables. The first two lines, both in iambic, have two stressed syllabus each. In these opening lines, each unstressed syllable pairs with the stressed syllable after it to form an iamb (or iambic foot, if you prefer). The fancy term for a line with two iambic feet is "iambic dimeter."
Or, to answer your question in just a few words:
Two feet per line, except in those two longer lines.