Want to make a 14 inch pillow cover

I have no pattern… i found this lovely soft fuzzy yarn that will match my girls’ room perfectly and I want to knit (garter stitch) a basic cover for a 14 inch square pillow form - no need to make it fancy - you won’t be able to see the stitches, anyway. I’m using size 11 needles and my gauge is 3.5 stitches per inch (from left to right - not up and down). i’ve measured the pillow and it is indeed exactly 14 inches from seam to seam - both ways. How big should i make it? meaning should i figure for the cover to be about 14.5 inches (which would be 50 stitches cast on) wide? bigger? smaller? and length of the piece to be about 29 inches? bigger? smaller?

thanks for any help you can offer.

Check your swatch to see how much ease it has (pull all directions) and determine if you want the cover to be loose or snug. If loose, then go with straight calc of 3.5x14 (or whatever length, as well). But if you want it to be a bit snug then I’d go down a few sts/rows (I prefer neg ease on pillow cover otherwise they can sag over time…sometimes). Did you launder the swatch to see if it will grow/shrink over time? (I assume the cover will be washed?)