It depends on how squishy the form is and how much you like plump pillows. I've made the covers 15.5" square to fit a 16" down pillow form and 16.0" square to fit a 16" Joann basic type form. IMO more than 0.5" smaller ends up looking like a size 16 girl squeezed into a size 4 dress, i.e., bursting at the seams.

I would start out by making them the same size as the pillow form. You can always take in the seam 1/4" all the way around until you get it just right.

I always pull the side seams of the muslin cover on the form taut, measure and use that measurement plus 1/2 " to make my covers. Hasn't failed me yet! HTH

Thanks for all the advice. I got an 18" pillow form at Joann's and sewed the cover exactly 18 x 18". They turned out perfectly but they weren't super fluffy pillows. For fluffier pillows, I think adding the 1/2" would be a good idea.

Congrats on your new throw pillow covers, Randita! You need to show us your handiwork now. As much as some people like the fluffier throw pillows, I like them to be less so, so that they kind of mold around your body a bit. My "too fluffy"/overstuffed ones usually end up on the floors, as my family says they're not comfy to lean against.

Yes, please, show us what you've made!

Here is a link that might be useful: Nouveau Stitch Home Dec Sewing Forum