Finding the resistivity of high resistivity liquids
This is to elaborate on Phil G's comment.

You can measure resistance between two large Plates, keeping the gap between the plates small enough that the total resistance stays within the range of an accurate sensor.  

The closer together the plates are, the more misalignment or roughness of the plates will decrease accuracy, so the smaller the volume you want to measure, the greater the machining cost.   

The greater the area of the plates, the easier it will be to align them, and the harder they will be to machine.  The sides of the device must have extremely high resistivity, preferably orders of magnitude above the material being measured, and I have no idea what would be available to meet that spec, so I'll just mention that because area increases exponentially with plate size and perimeter length increases linearly, the large the plate, the less resistive a border material you need for the same accuracy.