Well, to design a quilt, I think the first thing you have to think about is, what is the end use? Is it going to be a baby quilt? Is it going to get washed every other day? Is it something you're going to give to a new daughter-in-law? What's the use? Who's getting it, where is it going?
That's going to give you a lot of your information to start with. Then you're going to decide how big you want it. If it's a baby quilt, it's not going to have to be very big. If it's something that's going to fit on a bed, yeah, you're going to need more fabric. If it's a wall quilt, you're looking at it from a different angle.
Once you sort of get those parameters in mind, then you can decide about how many blocks, what kind of blocks? Is the quilt going to be pieced or appliqué? The nice thing about quilting is, if you don't want to design a quilt, there's many books, there's many patterns. It can be as simple as you need it to be.
However, if you like to do things from scratch and you want to put a lot of yourself into it, you could set out to design a quilt and you can do it with graph paper, and you can draw it all out. That helps you figure out measurements. There's even quilting software you can use and that's very helpful too. In the end, designing a quilt is finding shapes and patterns that speak to you, and it's a really enjoyable process.