At this time [3/6/15] the only ground based vehicle is a 'hover-vehicle'. Its limitations include - Surface ONLY travel just a few meters from the ground. The hover-vehicle is designed for planetary scouting.
The hover capability does allow one to travel across water however, cutting power to the hover engines while over a liquid body will result in the hover-vehicle then dropping to the surface and sinking.
It is not yet known if one will be able to use it as a 'mule' to carry additional ores/materials/resources.
Additionally, it is further unknown if hover-vehicles will be limited to small blocks only or large also.
Small ships are created using small blocks only. The size and shape of small ships are limited only by the players imagination however, Gravity, fuel usage and other contributing factors mean there will be a 'limit' on overall mass. For example, one could not expect a single small thruster to lift a huge amount of weight into space.
Small ships are designed for in-system space/planetary travel. They may enter/leave planetary bodies for exploration, conveying resources to and from a planet surface and mother ship
Small ships will additionally be capable of combat.
Large ships are designed for travelling from one solar system to another. Mother ships may also be used for storage of resources and additional small ships.
Large ships will allow for the growing and storing of food items. Creating oxygen. Energy creation. Defence.