Forgive me if I am not understanding something but using D=1/2*rho*S*Cd*v^2, I would take k to be equivalent to 1/2*rho*S*Cd. Plugging in values and units I get .5*1.229 kg/m^3*2.6m^2*.45 (dimensionless drag coefficient) = .72 kg/m^3 * m^2 =.72kg/m. When multiplied with the v^2 term, I get units of kg/m * m^2/s^2 = kg * m/s^2 which is of course equal to a Newton (appropriate for a force).
I am plugging in values that I would expect for something like an automobile. I believe 1.229kg/m^3 is an appropriate value associated unit for density of the atmosphere at sea level.
Plugging in v=20m/s, k=.72 and an x,y scale factor of .01, I get an xmax=2.91e-3. I believe the value not acounting for air resistance to be just over 41m.
Thanks. Sorry for any misunderstanding.