Nellie Quats sizing runs on the generous side, although every child is different and fit is individual to each child. Please read the description of the product such as the length, 'oversized' styling etc to make sure you order the right size for your little one.
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. Often, pieces last beyond the age range specified.
If you are unsure of what size to go for, feel free to contact with any more questions.