Patterns often have measurement tables to refer to when choosing what size to make. Often the sizes are referred to by age or the height of the child. The various sizes will then have a list of key measurements like chest circumference, leg length etc. In order to figure out what size you should use, you first have to measure your child at the key points mentioned in the pattern's reference table.
Ordering fabric online can be like waiting for Christmas Eve. You know that there'll be a special present just for you. You even know what sewing pattern to use with it and you have made sure to have matching threads on hand. Then, when Santa Clause the mailman finally comes, you rip open the parcel and do a little dance of joy. You start to take out everything you need for your new project: the pattern, scissors, thread... but oh no! It dawns on you that the fabric hasn't been washed!