Electrodesizes in elctrolysis

Hello everyone,

for some experiments i have a kathode which is a 4-inches wafer with some fancy surfacetreatments. As an Anode i unsually use a platinum electrode. I use voltage instead of current densitys (i try to copy another project). The problem i have is, the anode is pretty small arround 2 mm^2 - at first this was not an issue, because my kathode was also pretty small. So, i'am sure this isn't a size ratio which is ideal, tbh i don't think this will work properly. Is there a formula or some theory, which gives me an idea, how big a platinum electrode should be (or how voltage and current density is influenced by unequal elctrodesizes)?

And something else, which doesn't fit too well here, but maybe someone knows: In case of an insufficient platinelectrode size, i have the possibility to use evaporisation deposition to create a platinelectrode of the desired size. The thin film would be arround 10- 100nm (or more if needed). But i still struggle to find a substrat which does the job - it shouldn't be isolating (like a Si wafer wich forns isolating SiO2 on air) and it shouldn't be metallic (i fear to create a weird alloy of platin and the metal). Any good ideas?