G'day Sossy and Norm,
Just had a bit of a scratch around in the shed as I've just dismantled a parts beast. I'm going to take a punt and say your nut size is 1/2 inch NF. I say punt as I've nothing here to measure it with but a steel rule. I can take it to work tomorrow and do a proper job.
I think it's a fair bet. They were basically an imperial machine. Make sure the nut runs on fully with fingers before using a spanner to ensure you have the correct size and are not cross-threaded.
The wheel is notorious for corroding/seizing in the axle. If you've been driving it around without a nut, that well demonstrates the extent to which this happens.
Cheers,
