the knives will stay firmly attached to the mag strip if its a good one like you get in the tool department... the ones I buy from harbor freight only cost 4.99 and they are as strong and as good of quality as you will find elsewhere at 3 times the price... they are about 15 or 16 inches long.
we use the mag strips for knives in the RV, and one just inside the door, for holding tools so that they are handy to get to either from inside or from standing on the ground outside... and one in the basement to hold assorted tools that need to be accessed frequently..
and we have a strip in our boat for holding the galley utensils such as knives, can opener and bbq tongs....
they have proven themselves to work well for many uses, and it will take some real road abuse of the trailer to shake a kitchen knife or set of tongs from it...