If you want to make a flower with sixteen-petals, you need a paper with regular hexadecagon shape instead of square. This page shows how to make an origami regular hexadecagon from a square paper.
In this instruction, I used 15cm x 15cm origami paper. Of course, the folding method is independent of paper size.
This is the front side of an origami paper.
This is the back side of an origami paper.