Whether you’re planning to buy a trailer to fit a new boat or to transport a boat you already own, make sure the trailer is big enough to accommodate the boat comfortably, but snugly. This requires you to know not only the dimensions of the boat but also the dimensions of the boat trailer. How to measure a boat trailer?
Step 1:
Measure the distance from the bow eye to the transom. This is very critical. Because the boat needs to fit in the space provided by the trailer. Without a suitable space, it could hit the back of your vehicle while turning, or be struck by someone else’s vehicle.

"A" Bow Eye to Transom Maximum
"B" Bow Eye to Transom Minimum
"C" Rear to Angle Support
Step 2:
Measure the maximum outside width of the trailer. From sidebar to sidebar, or from fender to fender. The purpose of this step is to ensure that the boat will fit snugly against the trailer.
Step 3:
Measure the vertical distance from the ground to the top of the trailer. Because when you are putting the boat in the water and loading on the trailer, you need to back up on the launch ramp. Taller trailers need to go further down the ramp. For example, a 3.5-foot trailer will need to travel farther into the water than a 2-foot trailer.

Step 4:
Look at the trailer placard that shows the Gross Vehicle Rating (GVWR), calculates the weight of the boat that can be carried, and the amount of equipment. You can also calculate the capacity of the trailer according to the weight of your boat and the equipment you need to carry. You can refer to the following formula:
Boat weight + engine weight + (fuel capacity x 7 lb / gal) + (water capacity x 8 lb / gal) = Total
Then
Total X 1.1 = Trailer Capacity Needed