Question: How big a Star could be?

[BlurryVortices]

The most massive star we've ever been discovered is RMC 136a1, With a mass of 265 M☉.

But my question is this: How massive a star could be? Without turning into a Black Hole or Become a Hypernova?

[Magnetarhyper4436]

Stars currently in formation can collapse into black holes straight away if their mass weighs down too much. So with massive stars it depends on how dense they are and if they hit their schwarzschild radius or not (depends on both mass and radii).

[Magnetarhyper4436]

Also VY CMa isn't massive, it is simply large and the largest star is UY Scuti which has even less mass than said star! 

[Chaou]

There are some difficulties in making massive stars, one of them being that especially massive stars only tend to last for a few million years or so. Massive stars don't hold onto their mass very well either, and will often wildly eject it's outer layers to reveal layers of elements heavier than hydrogen.
These stars are called Wolf-Rayet stars, there's also the issue that if the star making gases are too hot, then instead of collapsing to make a star they could just disperse. R136a1 apparently defied alot of previously established models to do with this, but I did find some links to research papers that attempt to explain this.
http://adsabs.harvard.edu/abs/2012MNRAS.426.1416B
http://arxiv.org/abs/1302.2021v2
http://iopscience.iop.org/article/10.1088/0004-637X/732/1/20/meta;jsessionid=3590EF7639F2C755C1C0C48D259A6CBB.c1