I'm confused... people say the is a single universe. If the universe is fixed in size wats there outside our universe?? can't there be another nos. of universes??

First, the unverse is not fixed in size - it is continuing to expand. The confusing part is that it is not expanding into empty space, because there is nothing "outside" the universe - it contains all that here is - but rather it's the dimensions of space itself that are expanding. It's not like a balloon hanging in a room that can expand to fill more of that room, since it doesn't make sense to talk about what's inside the universe versus what's outside the globe. As for the question of whether thee are multiple universes - there are certainly theories along those lines, but their existence can never be proved (or disproved), as by definition these alternate universes can never be observed and can never have a direct impact on our universe. So these are merely interesting conjectures that seem to be the consequence of various cosmology and quantum mechanics theories.

As ebaines said, and I would like to reiterate. The universe is all that there is. It is everything. There isn't an "outside" of the universe, only an inside.

Yes, it's very hard to get your head around :). It is possible to have a shape which is boundless but finite.

Einstein once said: "If we don't look up at her, will the moon still be there?"

The same can be said for everything - things don't become existent until we view them. If that is the case, then everything around us is as we shape it, though we lack the skill and knowledge how to control that.
Research stemming from Einstein's Zero Point Theory has already found hints of an all-compassing energy field present in everything, of which perhaps everything is made: planets, people, trees, memories, past, present, future. If that were ever proven, it could fundementally change everything we believe in.

And perhaps end everything.