The length and fullness of a petticoat or crinoline is a matter of personal preference. From a softly gathered prairie length nylon to an eighty yard lamé and lace creation the choice is yours. The proper length is about half an inch shorter than the skirt. If the full slip is too short the shirt will 'lampshade' or droop over the edge. A prairie slip should be very close to the skirt length. The purpose of the slip is to prevent the skirt from tucking between the legs and to reduce shadows showing through the skirt.
I do not recommend making your own petticoat. I have made a couple of them and the effort is not worth the rewards. One of my friends told me "Sew the easy stuff - Buy the hard stuff". Save your time and energy for making dresses and shirts. Petticoats are available in a hundred colors and different fabrics. Colors and fabrics that you will not be able to find in your local fabric store.
When contemplating the purchase of a new petticoat don't get with basic white to 'go with everything'. Instead get that wild bright color that you just love and make your outfits go with the petticoat. This purple petticoat could be worn with many different prints. To a solid pink dress, add a few purple ribbons. On a black dress, wear a purple belt. For a green dress, wear some purple beads. Petticoats are for fun. Wind up your wild side and get the brightest color that your heart desires.