I went back & read this again....."I have for the envelope pillow called for a piece of fabric 19x15. I wanted a smaller pillow and wanted to reduce the long side to 15. "
It doesn't say ....reduce the entire pillow in size....just the long side.
So if the fabric is 19 inches long x 15 inches wide & she wants to reduce the long side to 15 inches (assuming before sewing) then cut off 4 inches from the long side. End result would be 15 inches x 15 inches cut fabric. Take off for seams & the pillow would measure 14 inches x 14 inches finished, using 1/2 inch seams.