In the trade office the merchants’ carts get improved and equipped with powerful horses. The higher its level the more your merchants are able to carry. With each level of this building, merchants will be able to carry 10% more resources.
The Roman Trade Office provides a bonus of 20% per level, rather than 10%. This bonus only applies to the Roman trade office.
The level dependent attributes (such as construction costs, generated culture points and merchant capacity) can be found here.
For the full table of construction times click this link for server with 1x speed and this link for server with 3x speed.