Right now i'm trying to learn "Don't think twice, it's alright" By bob dylan, fingerpicking style and numerous times you are required to reach your thumb over the fret and press down the low E while doing C and F variations. (if i made any sense right there, i'd be surprised)

but i'm just not able to get my thumb over enough to press down the string clearly, ive tried multiple different ways of holding the guitar and i still just can't do it...

is there any way to be able to overcome this?
Guitar player of 9 months and counting...12/6/09
You might need a guitar with a thinner neck. What are you using right now?
being short is good for nothing! for real...if i could get surgery on my thumb i would haha, is that possible?!

and right now I'm using this crappy guitar i got at a pawn shop, its my first guitar...I'm soon to be getting a new one, but its a "ventura"
Guitar player of 9 months and counting...12/6/09
Give up....

or find a way that works for you...you are not Bob Dylan...
In the Valley of Vung
I think its more down to flexibility than actual hand size - when I first tried to use my thumb over the neck I could hardly even mute with it, and now I can comfortably fret with it I'd say if you really want to be able to do it keep trying and it should get easier - but don't leave it there all the time, cos you'll have much better hand position and reach for normal playing with your thumb behind the neck
I'm 1,72m (about 5'7.7") and i would say my hands are not big, and can kind of do it on my steel string (1,72 inch / 43,7mm nut), but no way in hell on my classical.

I'm not an expert at all on Dylan (although i want to get into his music), but i'd assume he's not playing a classical.
the only option is a smaller neck. there can be a huge difference in guitars. if that dont work, just practice something else :p
Don't use your thumb. Putting your hand in that position (like your holding a bat) can cause tendinitis.
I know what you mean about having a hard time wrapping your thumb over the next to grab that bottom E string, I've never been good at that either. You can usually always figure out a way to re-finger those chords either (as chainsawguitar suggested) using a barre somehow, or by just using alternate fingers. I've been playing for over 20 years and have yet to run into a scenario where you absolutely HAVE to use your thumb as a wrap around. Just analyze the notes in the chord and see if there's another way to voice it - maybe at a different position up the neck.