When booking your journey on the "Add a vehicle" page you can select whether you are travelling with a trailer. Here you can select if you are travelling with a:

Low Trailer/Caravan: <2m length; <1.83m height

Trailer/Caravan: <6m length; Any height.

Trailer/Caravan: <8m length; <4m height. Enter length if 8.0m or greater.

Or other size, where you can input your own height and width.

Once you have selected your trailer size you can carry on with your booking.