Usually when I write i get really into it and can pop out one or two stanzas with really colorful language and imagery that i really like. When i try and go for a third verse or write a chorus, my imagination sort of shuts up and everything that i write after that point seems crappy and out of place in comparison to the first few lines that i wrote.

Other times its like i cant shut up and by the end i feel like i wrote an epic poem that's completely overwhelming bordering on boring. Should i keep my songs based on those few lines or stanzas that really work? What should i do if i want to expand the lyrics to fit a longer musical structure?
Go progressive.

Or write whatever you want to and try to adjust or/and rewrite piece by piece.
Exactly what my problem used to be and still is at times. What I try to do is when I write a song I feel is too short, I take a line I like and try to expand upon it, maybe lengthen the stanza or chorus. If you feel the song is too long, try expanding it to be 2 or more songs. Just remember that songs are written for your feelings or beliefs, so if you can only punch out a few stanzas, or it is a page long song, it doesn't matter as long as your got your point across and you are happy with it. Look at songs that are 20 minutes long (Progressive), and songs that are 3 minutes long.
This is one problem I had when I first started, is I'd try to write songs that were too long and give myself writer's block by trying to write too much. Then again, there's one gigantic song I wrote from that time that I'm still very happy with. So maybe longer songs are you're thing. It also depends on what kind of music you're writing for. Long songs tend to make more sense for progressive rock or doom metal than hardcore punk for example. At the end of the day, you just gotta do what feels right for you. Just keep in mind that a listener's attention span can only stretch so far.

As for songs that feel too short, what I usually do is repeat the first verse as the third verse instead of writing something completely new. That way, the first verse gets reinforced in the listener's memory. Of course, that's assuming the song needs a third verse in the first place. Sometimes, two is enough. You just have to listen and decide what sounds best to you.
Just make each song the length it comes out at. Don't force it to be longer or shorter, when you're finished writing, stop.
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