Bonjour Atomillo:
Just reading this long thread now, we have used since 1980s high gig R and small C.
But You did not mention the source of the current or system setup. Still we can comment:
1/ Your root goal, measurement of small currents (pA-fA) is best handled by an electrometer, the old Kiethley 610C are designed for such use and have the appropriate gigohm R and very low leakage input connectors and amp JFET.
https://122.physics.ucdavis.edu/course/cosmology/sites/default/files/files/Ferro%20Electricity/Keithley610manual.pdf2/ A DIY electrometer needs a special input amp or opamp, and teflon connectors. The feedback element can be a metal oxide R available from Victoreen, Caddoc, and other high R vendors. Expect 1 % accuracy at best.
3/ pF capacitors are not made in SMD nor at high precision. GenRad made some large and costly precision air cap standards, available on Ebay, ~$100...500
4/ Any pF measurement must be a 3 terminal guarded bridge or RLC meter, with no guard the measurement is very inaccurate.
5/ Slight lead position changes will dominate the test of pF C, you need a ridgid test fixture, with guard, and you must first zero out the test setup stray capacity.
6/ We measure with vintage Tektronix 130 valve LC meter, 3 pF lower range up to 75 pF zero capability.
Another good meter is the HP 4332A. They have the required guard voltage capability.
7/ The capacitor is fabricated with metal spheres (NOT PCB!) and teflon or delrin, nylon insulators as per good high voltage technique. One simple low capacity topology is two equal diameters spheres, like a spark gap.
8/ The formulas for capacitance are well known, and calculators available online for parallel plates, spheres, single sphere above ground plane, etc.
9/Our calibrated HV sparkgap is a pair of 50 mm metal sphere son 5 mm rod supports , 30 mm dia Lucite columns supports 200 mm spacing. We read ~ 2pF guarded C between spheres with gap is 15 mm
10/ A parallel plate capacacitor of PCB matériel will have higher C than calculated for a metal plate due to the PCB insulation layer diélectrique, eg FR10 fiberglass. You must account for the electric field on the back of the plates.
Hope this note is interesting,
Bon chance,
Jon