Press Select to bring up the guide. Press Triangle to active Boy. Then proceed to eat (L2) the words Noby Noby Boy!
(Tip: Eat the words one letter at a time, in order, and try not to get any other letters besides NOBY NOBY BOY, if you mess up press select and then press it again to get back into the guide.)
(Thanks Cory Newb)

Hold right on the Left stick and press Start to bring up your real-time stats page with your length. Keep stretching and then shrinking to build up your unreported length. Now watch it very carefully and go very slow once you get around 750m. Once you see that it says 765 get out of that menu and press up on the Left stick and press Start to bring up the Girl menu. Report your length and the trophy will pop up.
(Tip: If you are on a map with any kind of moving objects, i.e. cars, if they hit you while you are close to 765 they will cause you to stretch more and you have to start all over.)

You can get this trophy at the same time as the '20' trophy. Just stretch out as far as you can, then continue to eat everything in Boy's path (Doesn't have to be inhabitants unless going for '20'). Once you are completely filled to Boy's Head press and hold L3 to shrink from Boy's head to his rear (Shrink Completely). Boy will be a little fatter and start farting/pooping objects out, so before he lets it all out stretch back out as far as possible and repeat the above process about 6-7 times (Maybe less depending on how far you stretch and eat.)
(Update: A lot of people have said that they had to wait for Boy to fart/poop everything out before stretching back to max and eating more. So if my may doesn't work for you, try this way.)
(Thanks Scotsman-15)

Do not have anything in Boy's stomach. Eat an animal, i.e. dog, and a person. Then press and hold R2 to fart/poop them out of Boy's rear end. They will be combined and the trophy will pop up.

You need to get 10 people/animals on you at one time. My only suggestion is to stretch out somewhat far and go really slow as to not knock people off. Try to avoid hitting other objects as well. The 10th person will make the trophy pop up.
(Tip: Find a level with a lot of Little People, stretch out, and just sit near a big group of them. You should be able to get it.)

Pretty simple really all you have to do is stretch yourself out enough so that you can fit 20 people/animals inside you at the same time. Once you get the 20th the trophy will pop up.
(Tip: You can count the number of inhabitants inside boys stomach, just count the humps.)

You can do this right when you start the game just keep stretching Boy until the trophy pops up or you can hold right on the Left stick and press Start and that will bring up your real-time stats, like current length and unreported length so that you can track your length easier.

When inside Boy's house go to Quit Game and Fairy will come out and then Bye-Bye Boy will come out of the chimney. Congratulations now you have to go back to XMB and restart the game if you want to keep playing.

When inside Boy's house press down on the Right stick and it will rotate the roof.

Hold up on the Left stick and press Start to take you to the Girl menu. Here you need to upload your current length to Girl. When you do that you will see the little squirrel pop up in the bottom right corner. Shortly after you see him the trophy will pop up.

First thing you do in the game is take a quiz. I do not believe you can get a question wrong. Fairy will just ask you a few questions on controls (like your gonna know it once you start). Believe there are only 4 questions.