I am 6'2" and have size 17-18 feet (abnormally big for my size). My feet have grown at the following rate (though I have been the same height since I was 15.

Age 15 - Size 13
Age 16 - Size 14
Age 18 - Size 15
Age 21 - Size 16
Age 25 - Size 17-18

My question is the following... I have heard that there are several health conditions where abnormal foot growth is considered a factor. Could anyone let me know if there are conditions I should be looking into? My feet are shaped normally, but I have had serious problems with athlete's foot in my life and burning and numbness of my feet. If anyone could help, I would be so appreciative. Thanks.

Large hands, feet and facial features can often be signs of a pituatary dysfunction. The athlete's feet does not have anything to do with the size of your feet and I suggest seeing a podiatrist. Often over the counter medications are not as strong as a perscription or you may have a bacterial infection and not a fungal infection.
Burning sensation is often a sign of nerve damage which can be caused by diabetes, vitamen deficiency, alcoholism, AIDS and others. I podiatrist would be able to adress this concern as well.