Trucks control the turn radius of your deck. Ideally, trucks should be the same width of your deck. Setting up a complete with trucks that are too small will feel unbalanced. Building with trucks that are too large will feel unresponsive. Generally, 150mm trucks are used for cruising, slalom and carving. 180mm trucks are used for downhill and freeriding.
Trucks come in different measurement types. If it is listed in inches, it is showing the axle length. If it is in millimeters, it is describing the hanger length.
The rule of thumb is as follows:
If the deck is under 9" = 150mm or 9" trucks.
Over 9" = 180mm or 10" and up.
Want to know more? Read about truck baseplates here.