Load is the number of units (boxes, meals, flowers, items, etc.) associated with a delivery stop. Capacity is the maximum number of units a vehicle can carry at once.

In each stop, you can enter the stop's load. 

If you're uploading your stops as a spreadsheet, just add a column in your spreadsheet titled 'Load.'

After indicating loads for each stop, you must also indicate capacities for your drivers:

Now leave it to Routific to ensure your drivers are not assigned more than they can carry.

You could assign a numerical number to each box size. For example, a small box could be 1, a medium box is 5, and a large box is 15.

Then, make sure to calculate the capacity of your vehicle correctly (ie. how many small, medium, and large boxes it can carry in total), and plug that into the driver's capacity field.