There are no theoretical limits. Magic can, given enough of a power source and a properly constructed spell, do literally anything. The Laws of Magic are a simple enough guide to the possibilities; magic can warp minds, alter flesh, kill by the droves, and even rewrite time and space.

Once active, however, a spell still has to work in the "real world", so to speak. Physics plays more of a part than you'd expect. For example,  let's take Harry's favorite fireball. You can build that spell to just directly create heat for the fire, but that's pretty exhausting on the magic. You could also build the spell to just move nearby heat around, stealing it from other sources to make the fire hot. That's a lot less intense on the magic, but might make it harder to do in a frozen tundra.  Point is the heat for that fireball has to come from somewhere.  Magic doesn't provide a free lunch, just lets you bend how to pay for it.

If you wanted to do something catastrophic, like alter history, the energy involved becomes insane, fast. Really, time only flows in one direction,  and we haven't figured out any rules that change that on Earth.  So every ounce of power you get would have to fight the natural order of the weight of time, just to get into the past,  even slightly. Then the spell would need more to change whatever you needed,  more to make things flexible enough for the timeline to accommodate, more, more, more. 

So practice reigns rather than theory and the effort involved in bending the world to your whims usually wouldn't work out. A single wizard can only channel so much power on their own. Gathering more would require focuses for something elses energy, places to store the energy up, preparation,  more time, etc.

The biggest magics seen actively so far in the series -

 >! Kemmlers Darkhallow and the Red Court's bloodline slaughter

are usually shortcuts past all that, using some form of human sacrifice to harvest life energy by the truckload.

There are beings that can break the practical boundaries,  but by and large they exist under their own laws and natures (or laws of nature) and are constrained from large scale meddling.