I've not used my track saw a lot (only got it mid November), but found the 24" to be a pretty decent length to start with. I'll add a set of 18" once my credit card cools off from the Christmas madness!
I'm surprised Slappy did not mention how many Woodperckers rulers he owns, since he was responsible for me seeing the light, and trying to keep up with him on purchases. They work great, with the add-on stop, for precisely setting the stop on the Incra T-Track. I do not even use the built-in scales on the T-Track since they can slide a bit, and using the Woodpecker ruler is so much easier.
I bought the 36", then 12", then 600mm rulers, since I use Imperial at times, and metric at others, depending on the job. I suppose I'll round out these with their Imperial or metric counterparts at some point.