These are the sizes I've gone with, and once you figure about the size you want, you need to figure out how many blocks and rows you need to make around the size you want. I went with 90” long x 70” wide, on my leopard print quilt, and it's perfect for my double bed. It's more like a bed spread. You can also measure and get an idea of what you want, with a measuring tape. This is my Fun & Done youtube playlist, this will help you on getting this style of quilting down pat.
You can see from this screenshot it would give me 40 blocks, which is perfect. I needed 3 yards total for one of the fabrics for the top to cut out 32 blocks. The contrasting fabric was also for 3 yards, and I cut 31 of those to total the 63 blocks this project called for. Now I did Fun & Done style, so my backing fabric was a different story, I needed 8 yards of it for 63 blocks of 12” x 12” squares. I came up with 288 for 8 yards, for the width, I took off 8” and input 280 in text input for the calculator. It gave me 69 blocks of 12” x 12” so I was able to get what I needed out of 8 yards of black backing fabric, to do one of these two quilts:
You can see here I can get several more than I needed, so At this point, I was ready to and did buy my fabrics. You have to prepare them, and I believe washing with some white vinegar is useful to help the dyes not to run, and so I do that; you might not, you can use color catchers, or whatever works for you. The point of this tutorial is to help you get confident enough to buy that fabric you need to make a quilt this way. I've done a tutorial on Fun & Done already, so I won't rehash this, but you have to think about what you want in the middle of your quilt. I used flannel in one, and it was too light, even for Florida, so in my leopard one I used 100% cotton batting, the Warm'n Natural and it was great. Use what you like, and use coupons if you get it at JoAnn's, I've bought batting from Fabric.com as well as fabric, for my backings. Don't let the math keep you from attempting a quilt if you have the desire to do so. Just use that chart at the beginning as a guide. My measurements didn't match exactly what they had there, but I wanted 10” blocks and to use “X” styled quilting. You can do free motion quilting (FMQ) or whatever style you like. But the thing with QAYG is that you don't have to manipulate a huge pile of fabric and try to quilt it after the fact like in traditional style quilting where you piece together a top, then you add batting and back fabrics underneath your top, pin or spray basting it and then quilting it all together. To me that is so complicated I knew I couldn’t do it, so when QAYG was brought up, it peaked my interest and I hope I've helped to interest you in trying it.