I believe the judge often rides in WH classes so just wondering what size irons and length of stirrup leathers should I have to make sure they can accomodate the judge as I only normal have 4.25" irons.
Well, I'd have thought that standard length stirrup leathers (around 56/58") would be fine - same for irons. Apparently 4.75" is the average iron size for ladies so this would probably be best (around 12/12.5cm) because even though a man may require slightly larger, this size should be fine and your irons are technically child's so you wouldn't want to jump to gents! I was bought some 13cm gents stirrups accidentally once and they were huge compared to my 11cm old ones.
I've always used 4 3/4 irons and I've actually never thought about it, but never had a problem with it, many varied size judges have ridden in them without problem!
I ride in 4 3/4 inch stirrups and have only ever had one judge complain. He's got feet like canoes!
On the serious side, it might be worth your groom bringing some 5" stirrups into the ride; as it will be dangerous for you to ride in such big stirrups, due to the potential for you getting dragged.