Where can I find good information about maximum lengths supported? I found a good basic introduction, this doesn't mention lengths though.
It depends on the capacity of the used wiring and the speed you want to drive on it but in practice it's about 0.5m (at least for I2C, I don't have experience with SPI buses longer than a few cm).
I would like to use RGB, that would be four pins. However if I use 5050 RGB leds with a WS2812 I can drive them as SPI, needing just one wire.
That's not really an SPI, it's more like a OneWire connection. That's a possibility that may lead to a ATtiny85 as the controller of the node.
What do you suggest for protocol? I had a string of led lights that uses TTL levels for distances of 30cm between (20 lights). Or do I need something like RS485 (but that requires an extra IC)?
What are the requirements? Does it have to be 2 wires or are 3 wires acceptable?
RS485 is a bus technology that may be good solution for you but you're right that it needs additional chips. It's probably the easiest solution if you want to use standards.