If you use size 6 needles for this, it will be smaller than the pattern says it will turn out to be. You could cast on more stitches to compensate for that. It's not as if it has to be a particular size!
It says you should get four st to an inch and you cast on 25. So it's about 6 inches across. See how many stitches it takes for you to get to an inch an then multiply that by 6 and you'll get it to be that wide. Or if you want it wider, knowing how many stitches per inch you knit will give you a way to figure out how big it will be. Got it?