For resistance it depends on pressure, conductor and moisture of skin over the surface area. There may be a preference you have for pressure which affects contact area, so you may experiment with a DMM and record your range of values for dry and oil skin vs pressure for a fixed probe contact area. Don't use the sharp tips which will conduct thru the subcutaneous layers which have lower ionic resistance. These experiments may help. But essentially more trace width and area of contact will reduce resistance of skin, while closing the gap for insulation and contamination. The untinned copper will oxidize so you may choose to DIY etech or use organic gold plating or tinplate or hot air solder leveled HASL.
The reference circuit will work with a wide range of pressures and skin moisture levels. But for one person to use, less variability.
There are endless options for patterns but you want the finger capacitance to be much large than the trace capacitance. A 100 pF is nominal value for a finger tip ( from the HBM spec) unless you push hard, then x10. Sensitivity is up to you to define. Impedance is also up to you when you choose the frequency.
If you make the circuit signal to idle ratio 4:1 minimum with 1:1 hysteresis then you may want to define 100 pF for the finger tip and idle capacitance of 25 pF between traces. Geometry makes a difference and length of path for total surface area of finger to conductors.
It makes perfect sense to experiment and measure with a relaxation oscillator different pad designs for input capacitance and use a 1M feedback resistor with a good solder mask for insulation.
Saturn PCB toolkit V8.21.exe can tell you this in pF /cm for any coplanar gap and track width with a ground plane or not with some imagination.
You can make it a round target or two coplanar spirals or interleaved forks.
For contamination control, you can use a solder mask over the whole surface.
Carbon keypad buttons use a resistance which degrades over time. Your finger capacitance depends slightly on moisture but more on pressure and is very sensitive at higher frequencies used across the pads but no RF is need)
I have seen some students use pads as a DJ sound control and use rotary finger patterns to control volume or linear control for crossover and not just ON/OFF.
So use your imagination to define a spec, simulate it, make it, test it and measure it. Then repeat until perfect. (Or copy someone else's recipe)