You'll need to know three measurements (in millimeters) of your present fork.
- Brake Height is the distance from the centerline of the axle to the center of the caliper mounting hole.
- Ride Height is the distance from the centerline of the axle to the top of the fork crown, the base of the headset.
- Fork Rake is the distance that the front axle leads the imaginary line drawn through the center of the head tube (or steering axis).
Until you know these figures, you cannot know which fork to buy. As for these measurements of new forks, generally retailers won't have this information, or all they'll have is the rake. You'll need to visit the manufacturer sites to find out the height specifications.
Be aware that changing only the height without compensating changes in the rake will cause your bike to handle differently because of changes that will make in the trail. For an explanation of trail on handling, see
Trail and Its Effects.
I had a "high trail" bike and disliked the way it cornered. I used a new fork of different height and rake to make the handling neutral. I love the way it carves now.