What capacity would be a appropriate?
Its not the capacity but the type. Electrolytic capacitors are good for energy storage but have a lot of inductance and do not react "fast enough" for high frequency spikes.
Note: "fast enough" is not technically correct but communicates the characteristic.
Spikes come from:
- Ignition
- Lights going on and off
- Starting engine (starter + ignition)
- Shutting engine down
Like I said, your 1µF will likely be fine. Its just not what I would design into a device.