1. what is the least count of distance measurement that I can do with capsense ?
--> The basic principle of PSoC CapSense is to measure the capacitance of the sensor as well as to detect the change in capacitance of the sensor with respect to the ground. Basically the change in capacitance of the sensor that can be detected can be guaranteed up to 0.1 pF. So based on the change in sensor parasitic capacitance you can be able to detect the distance.
As per our recommendation please make sure that the parasitic capacitance of the sensors should be in the range of 5-45pF. Please note that you have to put the sensor dimensions in such a way that the minimum distance you want to measure will make a minimum of 0.1pF change in capacitance.
2. Which PSoC will be ideal for my design conditions?
--> You can go with PSoC 4000S/4100S device which contains the latest CapSense IP. The datasheets for those devices can be obtained from the links below.