does anyone know if the distance bonus granted to trade routes is based on the map size?
Sort of. You can have longer trade routes on larger maps, and longer trade routes have a larger distance bonus (up to a certain limit). The actual bonus you gain per tile of route length is unaffected, as far as I am aware.
Note that the distance bonus is flat, not a multiplier; a basic or slave trade route earns 0.025 credits per tile of route length (min 1, max 250), while a luxury trade route earns 0.05 credits per tile of route length (min 1, max 250). Note that the maximum value is essentially meaningless; the maximum distance between any two arbitrarily chosen points on any standard map size is about 761 tiles (maximum direct-route pathlength on an immense map; you might pick up a few extra tiles of distance going around some obstacles, but not enough to increase the pathlength to the 5000 tiles required to maximize the distance component of a luxury trade route or the 10000 tiles required to maximize the distance component of a basic or slave trade route).
