Are stanzas in the same poem usually the same or different lengths?

There is no fixed rule for stanza length in poetry. It's all about the feel and flow and so stanza length is one way to do it.

Consider for a second music lyrics - which are a kind of poetry. Most music is set to a 4/4 key signature and a repeating tune and so forces the stanzas to be the same length - in terms of syllables.

Poetry in the same length can be hypnotic or give the listener a sense of grounding - we can assume that the next stanza will be the same length as the former. Poetry of different lengths can add touches such as breathlessness or panic or haste or really any number of emotions - it's one thing to write that someone is feeling an emotion (which is what happens in prose) but quite another when it's written into how the words come together itself (poetry).

Other poetry is based on visual representation. Perhaps someone wants to write a poem that will feel very much like a block - and so chooses spacings and words that will give that effect (which is quite easy these days with textual justification tools, but handwritten it's a challenge).

Here's a link to a poem written in the shape of a circle (and is a palindrome poem (I tried copying it but the effect just gets lost - the justification didn't work):

http://www.poetrysoup.com/poem/circle_of_life_754462

Limericks are poems whose stanza length very definitely varies. As do haiku.