[Grant-53]

Once you determine the maximum effective RPM for your engine (when it stops accelerating in a lower gear) and the diameter of the tire you are using, the calculations will tell you road speed. Allow for tire squat by multiplying the diameter by 0.95.
Your riding position determines your shoulder height and the top of the main shell. The optimum body shell would likely be a 3:1 airfoil shape with a Kamm tail (0.85 x length) and curved sides. See the classic sailplane designs and the HPV body shell design software at recumbents.com.