Measurements of each garment are displayed on each product page.
Sizing generally runs true to size. Please also note the description of the product such as the length, 'oversized' styling etc to make sure you order the right size for you.
A size 18-24M is a 24M for an 18M to grow into, a size 3-4Y is a 4Y for a 3Y to grow into and so on.
The model shown wearing most of the outfits is age 4 wearing the size 3-4Y.
Feel free to contact with any more questions.