This work experimentally and numerically investigates the effects of fuel specific chemistry and transport properties on the critical flame initiation radius and associated critical strain rate. To isolate the effects of chemistry from those of transport, two sets of fuel comparisons are performed. Transport effects are varied without altering chemistry by comparing n-decane with n-heptane, chemical kinetic effects by comparing the C9 alkylated benzene isomers, 1,3,5-trimethylbenzene vs. n-propylbenzene. Measurements and simulations are performed using outwardly propagating spherical flames in air at 1 atm and 400 K preheat. A considerable kinetic effect is identified as a lower critical strain rate and larger critical radius for 1,3,5-trimethylbenzene as compared to n-propylbenzene. Transport effects in n-alkanes appear only for the critical radius, but were minimal for critical strain rates. A linear relationship is found between the critical strain rate and the maximum OH radical in the stretched flame, confirming that fuel specific chemical kinetics and its subsequent impact on radical pool population drive the behavior of the critical strain rate at the conditions investigated. A radical index for premixed flames has been further derived based on the rate of OH production and incorporated with transport-weighted enthalpy to interpret the behaviors of the critical radius for all tested fuels. It is found that that the critical radius is a fundamental property of unsteady premixed flame propagation specific to the chemical kinetics of fuel oxidation.