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There isn't' one standard size. Best to take whatever of it you can to a hardware store and match it up.

bloke wrote:phallic jokes...?? whatever...

Hm, small font there ...

Your choice to just test what you have is probably correct. From my experience, the threads/shaft on a ball come in two sizes. The light duty is about 3/4 inch and a fine thread. The heavy duty is about one inch and a coarser thread size. The hole in your hitch will tell you which one to properly fit it.

IIRC, the UNC (Unified National Coarse) thread for a 3/4" bolt is 10 threads per inch.

That's the thread on my trailer hitch ball, FWIW - I assume there may be others.

bloke wrote:...so riding mowers with worn-out/torn-up mowing decks have been converted to small (or not-so-small...in the case of the green one) "lawn tractors". ....

So.... is a mower a 4/4 and a tractor a 5/4? Do those come in silver?

If you tap that hole for the trailer ball and then sometime remove the ball to use it with the box blade, if the blade is attached by dropping a pin through that hole, you will have a good chance of boogering up the threads. The more use it gets with a pin in that hole, the more likely and sooner the threads will get boogered.

But based on what is shown in the picture, I don't have any alternative to suggest.