The size of your top will depend on how "stuffed" you like your pillows. I like a firmer pillow so I would make my top smaller. Just remember you can always make the top smaller so start larger and cut down as needed. As for seams, I would definitely use more than 1/4 inch, double stitch, and finish the seam edges. In my house the pillows are not there just for decoration.

Lots of helpful info. I've always just muddled thru when making pillows but it's not something I've done often. Might give it another try now.

I also like my pillow covers a bit looser so there is less strain on the seams during use and so they are easier to remove for washing. I usually measure from the center side of the pillow form to the opposite center seam then add seam allowance plus an extra inch.

I have a bit of experience making throw pillows. I always make the pillow form 1" all around larger than the finished top. This way, when the form is inserted, it will push on all sides and take care of empty corners. Use a 1/2" seam allowance for the top and in your case, the top should finish at 17".

One other thing I do is grade my corners, i.e., starting about 2-1/2 to 3" for the corner trim toward the corner removing about 3/4" when you reach the corner. Do this on all corners. I actually have a template that I use just for this purpose.) I do not like to see empty "rabbit ears" on pillows. Just me.