The key measurements on a fork is the diameter and length of the steerer. These both can be easily measured with a ruler. Also, if you change you fork length you will change you steering geometry.

Now for the obligatory BF two cents. If you change your fork to run 700c tires you will need to make sure you get a cyclocross fork so your brakes and brake levers will work. Also you will still have to run a 26 inch tire on the back. Since you already have a rigid fork you probably be best served by getting some very narrow 26 inch tires. You will not see much improvement by upgrading.