I used the same measurements for my 3 year old son and my 2 year old niece. The shoulder pieces were 2.5 inches wide with a 5.5 inch opening for the neck. The width of the body of the costume was 14 inches, the length was 16 inches. This size fit my 3 year old perfectly and was a little big on my niece, but this just means she will have some growing room in it. It wasn't falling off of her or anything so in my world it fit perfectly, lol.
I didn't do any measurements for the arm holes I just made them as long as I thought they would need to be for easily taking the costume on and off. I think, when in doubt make them a little bigger.
I used an 8x12 piece of tan felt to cut the stomach of the turtle. I didn't use a pattern, just cut it in what I thought was the right shape.
To give it some dimension I cut a piece of quilt batting to go underneath the tan piece of felt. I stitched the layers of the green body, tan stomach piece, and quilt batting in the same pattern as the Ninja turtles on tv.
I cut a piece of 2inch wide brown felt for the belt. I stitched it to one shoulder at an angle, then pinned it down and stitched the yellow buckle in place going through all the layers of felt,
I kept the shoulder strap not sewed down until I got the waist belt in place and stitched, this way I knew for sure how long the shoulder strap needed to be. After the waist belt was stitched down trimmed the shoulder strap.
Next I an extra strip of green felt to either side of the body pieces (right sides together). This is just so the costume hangs easily and moves freely. You don't want it to be in any way restricting.
For the shell I cut a random oval shape, cutting 2 pieces of quilt batting to make it more dimensional. For Grant's costume I used some polyfil I had in my stash to make his more rounded. The I stitched the layers together in the shell design I could remember from the cartoon. It's far from perfect but Grant knew what it was and that's all that matters.
I didn't sew the whole shell onto the back of the body piece. Instead I placed it where I wanted and stitched it by hand in 4 different spots. Add the shoulder strap by sewing it to the shoulder and stitch the waist belt on either side and to connect the shoulder belt to the waist belt.
Now stitch the front and back body pieces, right sides together, at the shoulders.
For the masks I made them about 5- 5 1/2 inches wide and freehanded the shape of an eye mask. I cut one out in green felt and the other, slightly smaller out of colored felt. I stitched both together then cut out the eye holes. I attached a 1 inch wide tie to either side.
And there you have it a Teenage Mutant Ninja Turtle Costume! These types of costumes are great for dress up play around the house which is what I originally made them for, however they are also great for Halloween. They are easy to have multiple layers underneath and kids can move freely in them. Grant decided to wear his for trick or treat and he had a long john thermal shirt, a sweatshirt, and a jacket all underneath his and it still fit well enough that he could run like a Ninja Turtle from one house to the next.
Grant won't hardly take his costume off and he and Sophie ran around my mom's house like ninja turtles when I put them both in the costumes. So score one for mom!