The size drive of a torque wrench has no bearing whatsoever on the amount of torque applied. The torque is measure at the end of the lever through the center of the device being turned. But in general the finer the torque measurement, the smaller the drive. Easier to make a 1/4 to tighter standards and to maintain the tolerances also larger drives will handle the torque being applied as well as the attachments. Using a drive size changer is not acceptable either as any flex between the components will distort the torque readings. The best small value torque wrench to use is a dial. Do not confuse that with a beam type torque wrench which has the pointer moving across the scale. Also click type, or the technical term micrometer torque wrench are designed to be best used by a typical repeating torque value and can lose calibration when adjusted often. Also never leave the click type set to a value other than zero as this causes undue wear on the mechanism as well. This is the torque wrench I have used for many years.
http://www.amazon.com/Torque-3002LDI.../dp/B000I1WPFE