What is the smallest touch sensation that a human can feel?
Short answer
The detection threshold of static indentation stimuli on the palm of the hand is approximately 10 to 40 micrometers, dependent on the exact location under investigation.  

Background
The sensitivity of a sensory system can be expressed as the detection threshold. This threshold is in psychophysics generally defined as that stimulus level where, on average, 50% of the stimuli are felt. 

There are several types of touch receptors that can be investigated, each with specific types of stimuli that are associated with them (Fig. 1)

  Fig. 1. Skin receptors and their response properties (Delmas et al. (2011)   

The question specifically targets detection thresholds of indentation stimuli. These stimuli are typically expressed as distance units (meters) or pressure units (Newtons). The skin indentation under constant force is dependent on the area of contact, skin type and other parameters of lesser importance. Hence, indentation units using distance metrics may be the most appropriate way of expressing static detection thresholds. Johansson &amp; Valbo (1970) investigated static detection thresholds on the palm of the hand (Fig. 1) and found average thresholds of 10 to 40 micrometers indentation. This variation was due to geographical differences; towards the fingers sensitivity is higher and thresholds are hence lower (Fig. 2).  


Fig. 2. Detection thresholds of static stimuli on the palm of the hand. source:  Johansson &amp; Valbo (1970)   

References
- Delmas et al. Nature Rev Neurosci (2011); 12: 139-53