Sheet materials will always be 8 by 4 feet, you won't find another size.
I'm guessing you want something reasonably cheap, as it's a playroom?
So you're probably looking at veneered ply or mdf, and a joiner to make a neat job of joining and stiffening the sheets to make a rigid top for whatever base you wish to use.
One nice alternative is birch ply, relatively inexpensive (45 quid/ sheet), rigid and a good finish on the outer plys.
Solid oak or cherry would be many times that price, maybe 400 quid for the boards, before work on making the tabletop starts.
The responses in this thread so far have largely been facetious and useless, and that's because, basically, you're asking for the impossible. On top of that, you're saying you want what would be an
*extremely* impressive piece of wood for merely a playroom. (If you'd said for a dining table in one of the panelled reception rooms in your Georgian house, people would have been a bit more serious!)
Personally, I'd seek out recycling dealers where I might find an old kitchen (or indeed dining) table, and then get that "done up". Don't bother looking in antique dealers (unless they're genuine junk shops), because an antique dealer will be trading on the value of such a table as you describe.
But "playroom" conjures toddler-sized kiddies: a huge piece of plywood fits the bill: let 'em get as messy as they want, and they can even paint it themselves every now and again.
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