1. I just take a digital picture of the trailer.
2. Measure something on the trailer that can be seen in the picture.
3. Load photo into my design program.
4. Make a skinny rectangle the length of the measured part of the trailer.
5. Set the rectangle near one edge of the measured part and lock the rectangle.
6. Scale the photo until the measured part is the exact size of the rectangle.
I usually use a door or an exact length for a number of rivets. Anything that you can measure and see in the pic. The larger the better. Now you are designing in full size.