A custom made space ship model that is 5 to 6 foot would be a insurmountable challenge unless you have built smaller custom ships in the past. An account of someone else who took on a project like this is here:
http://kotaku.com/custom-lego-star-destroyer-is-so-huge-it-has-a-three-le-1773324734
If you cannot find instructions on the internet, you can ask someone who has built one if they will sell you instructions or design you own using the LEGO Digital Designer software. To get the parts you would need to go on bricklink.com or start buying new sets that have the parts that you would need.
Quickly skimming the article I saw that it weighs 70 pounds making me think that it would cost at least two thousand dollars to get the parts you would need. Of course, if the color does not matter, you could probably build a FRIENDS-themed Star Destroyer for less. I think @paulamalu would love this project.