atomic radii order :
I > Br > Cl > F

F, Cl, Br, I belongs to the same group orderly. Atomic radii go on increasing as the nuclear charge increases from top to bottom in a group. The decreasing order of bond length C – I > C – Br > C – Cl > C – F. The order of bond dissociation energy
R – F > R – Cl > R – Br > R – I. During dehydrohalogenation C – I bond breaks more easily than C – F bond.So reactivity order of halides

$R - {\rm I} \gt R - Br \gt R - Cl \gt R - F$