Do you have a specific airplane you need to accomodate? Contact the manufacturer. He will have the dimensions needed to accomodate that airframe. (Then make things just a bit larger to make it easier on the people in the pointy end.)
The length and wingspan are important, but the wheelbase, gear width etc are important as well.
Also, remember the pavement design. You may have to improve/design taxiways and parking aprons also.