These are the averages of measurements that I found over and over. I have used many sources and my own experience to create this chart. The waist is the actual waist size, I cut my elastic about 1/2 inch smaller, and then overlap to sew about 1-1.5 inches. The waist is not too tight but totally not falling down. Plus, it gives it a lot of room to grow. The length is to the knee. Remember, this is the FINISHED length - so depending on how you make your seams, you will have to add inches to your pattern for the bottom hem, and the top casing for the elastic.
I can tell you that these sizes are pretty accurate based on my girls.
*Actual waist size, I cut elastic 1/2 inch shorted than the listed size, then overlap about an inch
** FINISHED length - add additional inches for bottom hem, and top casing for elastic.
Here are some skirts using these measurements
And, when making a tiered skirt - I like to use this calculator to figure out the measurements for each tier. But, you can also just use a multiplier of 1.5 for each row.