Bedfords Park is a very large park, but is it one of the largest in London? I've tried to find a definitive park ranking by using Google but it appears no such list exists. Wikipedia has a page on the Royal Parks and several subpages divided up by borough, but all written in very different editorial styles, some religiously specifying acreage and others very much not. The London Datastore didn't come up trumps either, and most of the supposedly relevant media articles were merely glossy clickbait. So I've had a go at making my own list.
I decided they all needed to be called Something Park, or be unequivocally a park, so for example Kensington Gardens (270 acres) would have been acceptable but Hampstead Heath (790 acres) wouldn't. The Top 10 might otherwise have included Ruislip Woods (726 acres), Mitcham Common (450 acres) or Wanstead Flats (335 acres), but none of these is a proper park in the parky sense of the word.
I omitted Lee Valley Park because even though it's technically 10000 acres it's more a collection of disparate riverside spaces than a de facto park. As for Trent Park and Old Deer Park I'm a little dubious about how publicly accessible they are so maybe they shouldn't be in my list at all. I also wasn't sure what to do about Osterley Park, which notionally has an area of 500 acres but only 350 are under the control of the National Trust and that's not all publicly accessible either, so I've left it out.
Then there's something massive called Wildspace Conservation Park (1580 acres) which is supposedly an area of Thames marshes and former landfill between Rainham and Purfleet. It was launched in 2006 with an eye to becoming a major leisure asset and was endorsed by Bill Oddie because it's prime birdwatching territory. But you'll not see the name if you ever go to visit, plus a fair chunk's not actually in London, plus it's hardly a park in any recreational sense of the word, plus I think it got rebranded Wildspace so technically isn't a park at all, so I think I can comfortably ignore it.
I'm not even sure that the acreages I've listed are accurate in all cases, they're just what Wikipedia told me, so they may be estimates or massaged or just plain incorrect. Indeed what I've ended up is about as undefinitive as it gets, but at least it's a list which is better than having no list at all.
At 215 acres I believe I can confidently say that Bedfords Park isn't in the Top 10 of London's largest parks. But it appears to be comfortably in the Top 20, along with Forty Hall Park (273 acres), Kensington Gardens (270 acres), Happy Valley Park (252 acres), High Elms Country Park (250 acres), Beckenham Place Park (237 acres) and Victoria Park (213 acres). Unless of course you know better, because what exactly is a big park anyway?