The trailer and towing vehicle should be designed to fit your boat.
Use the size of your vessel to determine the dimensions of the trailer needed. Today, most trailerable boats are sold as a package with a trailer of the appropriate size.
Look at the load capacity of the trailer stated by the trailer's manufacturer. If the combined weight of your vessel and its engine is more than 90% of the recommended load capacity, buy the next larger trailer. This is because your gear (fuel, PFDs, anchors, lines, etc.) will increase the overall weight by at least 10%.
Check the owner's manual of your towing vehicle to ensure that your vehicle is rated to tow the combined weight of your vessel, engine, and trailer.