
While there's theoretically no upper limit on how big a black hole can be, there are hard limits on how big they could have become by now. The universe has only existed for a finite amount of time, and even the most voracious black hole can only suck in matter at a certain rate. The bigger the black hole, the bigger the gravitational field and the faster it can pull in matter - but that same huge gravitational gradient means that the same matter can release huge amounts of radiation as it falls, blasting other matter further away.