I don't quite get your initial premise: "But if this universe existed, then a 'parallel' universe wouldn't be able to exist."
I don't see why not.  The existence of this universe doesn't seem to have any qualities about it which would exclude the possibility of a parallel universe.  If something does "phase out" of this universe or otherwise change states to a parallel universe, the qualities that are no longer in this universe would have no effect on us.
Your presentation of the finite size of the universe, however, makes total sense.  But I'm not sure how the two relate.  Could you explain?

if the theory is true and that does exist. IT would be interesting to think about things you didnt do that you wish you do impacted your life

Everything that exists in our universe that allows it to exist wouldn't be present in a parallel universe is what I meant. Size is one major characteristic, if we are trillions of trillions of light-years cubed, than the parallel universe would either be negative or nothing. Although, it can be argued that there are a large amount of factors present that can possibly support the existence of a parallel universe. This is just my opinion, I do take in consideration that there are many other theories that make just as much sense.

One therory of a parellel universes which goes against your size point is that parallel univeres are in differnet diemensions. I've heard this I think its kindve far fetched but hey who knows.

Sorry for double post and this is of topic but to Decadentdragon: what does "I want to be prince of Amber" mean?

It would make sence and would be interesting to study i think that the idea is very far feched it was introduced in a 1920 novel so it is unlikly but possible!

Strategy guy, Google is our friend.

http://en.wikipedia.org/wiki/The_Chronicles_of_Amber

Sorry I had no idea he was talking bout a book. I thought he had randomly made a name for something or other.