It is very late and I should be in bed but I thought I might be able to help you start to answer your question.

When I worked in a fabric store I was taught "twice the length, plus the sleeves" when determining how much fabric you need for an item. That is based on an adult and fabric about 114 wide. For a child you would most likely need less as they are smaller and you may be able to 'smoosh' parts of the pattern tighter on a width of fabric. When I buy for my girls clothes without knowing the exact design I want I still pretty much go with the above rule, knowing I have room to move.

As far as the skirt you want there are two ways to get flow. One is to take a rectangle at the length you want, but one that is much wider, then gather it using pleats or a thread gather. The other is to cut on the bias (45 degree angle of the fabric) which gives it a fullness and drape that is gorgeous. Think 1950s style skirts. You use more fabric with the second option and the first is absolutely functional when it comes to little girls so I recommend going with the rectangle!

You can find heaps of free tutorials and patterns online for what you want. I had a quick google and I think that the best pattern for you might be the 'Oliver and S lazy days skirt' (google exactly that to find it). It is a free pattern from the super talented and super cute kids pattern designer Oliver & S so I imagine it would be spot on construction wise. There is also a pattern called the "twirly skirt" by house on hill road that has great photos/walk through instructions. Based on your DDs age I am going to guess she might be about 110 cm?? The house on the hill pattern (which isn't too full but still has some twirl) suggests you need two pieces of 1m x 40cm

The only problem is that you need to decide if you want the border fabric or the 'all over fairy' design, as each fabric will require different fabric dimensions because of they way they have been printed. I had just typed up a whole explanation but it was probably way too confusing as it depends on which fabric you want to use! Basically the border print means you have to cut the skirt the 'other' way.

In the ebay search you pointed out, the images with the double border are simply the full width of the fabric ie 114cm, and the single border would be the width cut in half. You would need to make absolutely sure what dimensions you were bidding on to make sure you had the correct amount of fabric.

The two fabrics should both be simple enough to use - the challenge here is in deciding which one and then calculating lengths, and they both could look equally gorgeous!

Let me know if I can be of more assistance (maybe helping you work out how much fabric when you know which fabric you prefer or what style of skirt?), I love making clothes for my girls. Love making that item that is perfect for both them and me!