I had the Rider 14 that I was replacing.  I have very narrow feet, but wear medium width running shoes.  I normally get size 8 in Asics, but Mizuno runs smaller so I got size 9 as their shoes guide recommended.  My previous Mizuno was size 8 1/2, but like the size 9 fit better.  Diffinately get one size larger.  This shoe is very comfortable.