Among alkali metal salts, the lithium salts are the poorest...

Ionic conductance in solution depends upon the mobility of ions, which depend upon the degree (or extent) of hydration. Since charge/radius ratio is maximum for $Li^{+}$ , so it is hydrated to a maximum extent. Thus, the ionic size in aqueous solution, instead of increasing, actually decreases on moving from $Li^{+}$ to $Cs^{+}$.