The concept of dimensionless scaling introduces the possibility of det
ermining the required size for a fusion reactor based on data from a s
ingle machine. Specifically, all dimensionless quantities other than t
he normalized gyroradius rho can be simultaneously matched to reactor
values in present-day tokamaks. Experiments on DIII-D show that the e
lectrons and ions have distinct rho scalings - for electrons chi prop
ortional to rho, while chi proportional to rho*(-1/2) for ions. This
observation can unify previous results based on single-fluid analysis,
and, in addition, illustrates the danger of using such analysis to ex
trapolate to a reactor. The rho scaling, coupled with technological l
imits on the magnetic field strength, determines the minimum size for
a reactor. If these rho scalings found on DIII-D hold under reactor-r
elevant conditions, the confinement will scale unfavorably as rho is
reduced to the values expected in a reactor.