One year at RAGBRAI I noticed an abnormally large bike parked in towns on two or three occasions. It looked like a normal bike just scaled up to a larger size. Then one day I came upon the bike and its rider out on the open road so I was able to keep pace with him for a while and chat. As you might expect he was a big dude and while I do not remember the size of the wheels on his custom built bike I do remember that he told me they were built up with unicycle wheel components. In fact you can occasionally see people doing the ride with large wheeled “road” unicycles. And then there are the penny-farthing crowd. So you may not be alone and I agree with the others that there must be some shops on the ride that can assist you if you have trouble.
But more than that, please post a link to a picture of what you are riding!