...exceptionally large leaves may attain widths of 61 cm (24 in) (2).
In fact, the width of Joseph's leaf (34.6 cm across) is so far off the accepted maple-leaf maximum size that I have to doubt Guinness really contacted him or his parents. I hope there isn't a scandal brewing.
And by-the-way, the pic above is an absolutely unexceptional specimen of a big-leaf maple maple-leaf. The wife and I collected it at on Vancouver Island several years ago. Note that its almost the same size as the ones these children (and I call them that only because I can't prove they're not children) are claiming as records. Such claims are not harmless; they sully the big-leaf maple brand.
There. That felt kind of good.