A pillow case dress for children is made with the selvedges sewed together to make 1 side seam. With 45 inch wide material that would only be snug on most adults. If you have a piece of 45 inch material try it around your hips and see if it's too snug. If it is you'd have to go with the fabric width as the length and cut off the excess. So pick a fabric that can be used that way.

Lay the fabric out and take any pattern in your size to cut the armholes. Only cut them 3/4 of the length of the pattern because you will not be joining the armhole at the shoulder.

Good luck