Is there a function to model the resistance between 2 metal probes
I'm not sure that will work well; you probably won't see a factor 5 difference between 5 ml and 25 ml.  

I would suggest something capacitive. The relative permittivity of water is about 80 times that of air, so you should be able measure a difference in capacitance between two plates with water between them compared to air. So the capacitance will be a linear function of immersion level. The advantage of the (insulated) plates is that you won't get electrolytic effects.