Those are very pretty!
I was shocked at just how much material a pillowcase really took (new sewer here, have been at it less than a year). I've made some flannel ones I love, they're just a touch smaller than a standard pillowcase, but they fit my pillows fine, so I haven't worried about it. So far I've used flannel remnants from the JAF remnant bin (I LOVE their remnant bin here, I check it out every trip I make). Their flannel remnants are often right at one yard, so they work well.