Ball Size :All trailers at 6000 GVW or less use a 2 inch                                                                                      All trailers at 7000 gvw and up generally use a 2 5/16th inch 

Ball Height:   14 in tires require 20-21 inches top of ball to ground 
                        15 in tires require 22-23 inches top of ball to ground 
                        16 in tires require 23 inches           

Light Connections: Surge disc brakes are a 5 flat, 5th wire activated by reverse lights to shut off brakes when backing. ( most newer vehicles with tow package come with a 7 round which has a reverse wire in it,  therefore a 7 to 5 adapter is used, or you can wire a 5 flat , v5th going to reverse wiring usually a light blue wire in your harness.                                                                        
Electric Brakes require a 7 round plug  

Ramp Tips

As you start to descend the ramp and feel the weight of your trailer and boat pulling you backwards, HAULT, shift your vehicle into drive,keeping your foot on the brakes lightly, let the weight slowly bring you down to the water leaving you always in a position to move forward if needed. This technique will also eliminate any sliding specifically gravelled covered ramps.