I know it is probably a competitor site but
www.7thgencivic.com has a really good tire calculator on their site. (My dad has a 2002 Si) You give the original size and then the size you want to buy... and it will compare diameter, revs per mile, Circumference, radius, and sidewall height, and what % your Speedo is too slow or fast. It also has a visual to give you somewhat of idea how wider or taller.... very helpful