I am unsure about how to fully understand how long a foot is in poetry. HELP?
The key to solving your problem is remembering that poetry is written to be heard. Read the poem you are analyzing aloud - if necessary, record yourself reading it so you can then focus completely on the sound of the language, without worrying about reading it.
A "foot" in a given poem is a specific number of stressed and unstressed syllables repeated consistently for the length of the poem. There is no one length that applies to all poems, just as there is no one pattern for stressed and unstressed syllables.
To understand how long a foot is in a given poem, listen to the poem being read out loud. You should be able to hear, if the poem has been written in a regular format (not all poems have a regular rhythm and stress pattern), that some syllables are naturally more accented than others.
Two woods/ di-verged/ in a yel/-low wood/, And sor/-ry I could/ not trav/-el both/ And be/ one trav/-eler, long/ I stood/ And looked/ down one/ as far/ as I could/
The dark syllables are the stressed ones, some as a factor of which syllables are accented in multiple syllable words and some as a result of the rhythm and message being conveyed. In reviewing the pattern created by those accented and unaccented syllables, you should be able to see that the usual pattern for this poem is one unstressed and one stressed syllable in each "foot" of the poem.