Replying to Summer jib help

with summer coming up i have been considering building a summer setup. however, there is only one spot to put it in my yard and it is less than ideal. i figured out that if i built the setup big enough to get a good amount of speed, i would be left with an area that is approximately 18 feet long by 11 feet wide to put rails/land. i'm not too worried about the width, but the length is a big concern. is 18 feet long enough to have a solid rail as well as a landing? if not i could probably get an extra 5 feet, but i would have to sacrifice some of the quality of the ramp itself. would it be worth it?