Think of the universe as a bunch of marbles (galaxies). Those marbles can move in any direction and there's almost no limit to where they can move. Because of their existance tho, space has to curve around them or the physics aspect would be off and it would be unbearable pressure making life or the moving of anything nearly impossible. It can expand and contract with the movement of galaxies but at any given time it does have a definite size that is too large to be measured by humans. Like I said before, this edge wouldn't physical exist but it is more or less a spot of high gravity where you can't move any further.