Navigate to the Operations tab, and then select the Vehicles icon.
Here you can use the drop down to select the type of vehicle you are creating/editing. The blue Capacity* link will bring you to another window where you will be able to setup different types of vehicles with varying capacities.
Click the Create New page icon.
You can set the capacity for that vehicle based on its abilities/size.
You can also edit a previously created vehicle by clicking on that vehicle on the right side.
